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If you sell an asset (eg property) after you become UK resident then you would be liable to UK CGT.

The fact that you have been a long term non resident and even if you purchased the asset whilst non resident would not impact on the position. If you sold the asset as a UK resident domiciliary you would be charged to UK capital gains tax (‘CGT’) on the gain realized. The gain would be calculated on the uplift in value from the date of acquisition.

Therefore ideally you would sell any asset in a complete tax year of non residence. However if you do have to come to the UK you could think about crystallising the gain whilst offshore - it could then be free of UK CGT as you're a non resident. This would rely on a Revenue Extra Statutory Concession. ESC D2 states that

‘…An individual who comes to live in the United Kingdom and is treated as resident here for any year of assessment from the date of arrival is charged to Capital Gains Tax only in respect of chargeable gains from disposals made after arrival, provided that the individual has not been resident or ordinarily resident in the United Kingdom at any time during the five years of assessment immediately preceding the year of assessment in which he or she arrived in the United Kingdom…’

Therefore if you have come to the UK for the purposes of employment or another settled purpose you are likely to be classed as resident from the date of your arrival. In this case any disposals prior to your date of arrival in the UK would not be charged to CGT. Therefore one option would be to transfer the asset to another entity or a connected person before you come to the UK. This would be a market value transfer and the gain would be crystallized whilst non resident. The disposal of the asset would then realize only a minimal gain as the base cost of the asset would have been uplifted to the current market value.

You should ensure that you take detailed advice on this. If you are non-resident in the UK you are normally taxable on your UK income.  This  means that UK tax should be deducted from UK pensions.

There are specific provisions in double tax treaties that can impact on this, and in particular there is a distinction made between government and non governmental pensions.

(Note that National Health Service Pensions are not regarded as Government pensions for the purposes  of most Double Taxation Agreements.)

You would need to review any applicable double tax treaty with your proposed country of residence as any relief available depends upon the terms of each DT treaty. If a benefit applies you can then make a claim with HMRC for the pension to be paid either
 - without tax deducted, or 
 - with  tax deducted at a reduced rate of tax as laid down in the DT treaty. 

We have some articles that look at this in detail check out:

http://www.offshoretax.co.uk/uk-emigration/offshore-tax/how-pensions-are-taxed-when-you-go-overseas.html

There are potentially a lot of tax issues to consider on any emigration from the UK.

In terms of capital gains tax there won't be an exit charge (unlike in some countries such as Australia) for  individuals. However you'll need to consider whether you have any held over gains as well as what you're planning to do with existing UK assets.

You also need to think about any UK tax reliefs and whether a disposal whilst UK resident may actually be advantageous if the overseas tax regime does not have similar reliefs.

Income tax also needs to be considered especially in terms of establishing whether a repayment is due.

Check out these articles for more detailed analysis 

Yes you can, but you'll need to consider your sources of income in detai. By establishing non UK residence you'll be outsdice the scope of UK tax on your overseas income. Any UK income or gains are less straightforward. UK gains can usually be avoided if you're non resident for at least five complete tax years. UK source income is technically within the scope of UK income tax but in practice there are a number of exemptions for investment income such as bank interest and dividends. 

See these articles for more detail on the UK tax exemption for non UK residents: 

http://www.offshoretax.co.uk/uk-emigration/offshore-tax/tax-implications-of-leaving-the-uk.html 

http://www.offshoretax.co.uk/uk-emigration/offshore-tax/checklist---emigration-to-avoid-capital-gains-tax.html 

http://www.offshoretax.co.uk/uk-emigration/offshore-tax/how-pensions-are-taxed-when-you-go-overseas.html

http://www.offshoretax.co.uk/uk-emigration/offshore-tax/when-a-non-uk-resident-is-taxed-on-uk-property.html 

This is residence that is established under the terms of a double tax treaty - as opposed to under domestic law. It can apply in a number of cases but especially to the cases of deal residents (ie resident in two countries at the same time).

The treaty will then outline the rules that should be followed to assess which country you are resident in and this will then apply when you're interpreting the treaty (eg for employmenr income, pensions etc). This won't affect your domestic filing position and therefore even if you're treaty resident overseas you'll still be resident in your home country for domestic law purposes.

 Check out this article on treaty residence:

What is treaty residence and how to use it  

•  you are a Commonwealth citizen (this includes a  British citizen)

•  you are a national of any European  Economic Area (EEA) state. 

The EEA states include:  

Austria,  Belgium, Denmark, Finland, France, Germany, Greece, Iceland,  Ireland, Italy, Liechtenstein, Luxembourg, Netherlands,  Norway, Portugal,  Spain, Sweden, United Kingdom and also, for tax years from 2004-05 onwards, Cyprus,  Czech Republic,  Estonia, Hungary, Latvia, Lithuania,  Malta, Poland, Slovakia, Slovenia;