Firstly bear in mind that the act of leaving the UK does not make you automatically non-resident for tax purposes. You need to complete and return HMRC P85 and if you leave part way through a tax year (as opposed to at the very start) you would need to remain non-resident for several years for all your earnings to remain tax-free.

If you bring monies back onshore when if you take up residence again you are liable to tax, so you need to move monies gradually taking into account annual allowances. It is always best to plan a return to the UK at least a full tax year ahead.

Even if you are a UK resident you may keep monies in an offshore account, but will have a tax liability on any money that you take onshore.

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"Good advice is always certain to be ignored, but that's no reason not to give it." Agatha Christie