As an expat you are not liable for Capital Gains realised in the UK, shares, property, art etc.
As an expat you are not allowed to hold an ISA.
If you sold something (shares) and made a Capital Gain prior to returning to the UK you should not pay CGT. However the advice I have had is that if you did this in the tax year you returned to UK, HMCR could construe this as an attempt to avoid tax and tax you anyway. Again the advice I have received is to take the Capital Gain in the tax year prior to returning in this way you would be absolutely not liable.
Generally speaking being out of the UK for a full tax year gets you clear of any tax liability the taxman may decide you owe with the caveat above.
So leave on or just before the 5th of April and come back sometime a few years hence on or just after the 6th April would be the unlikely ideal scenario.