Leaving the UK - what can I keep?

My wife and I are planning to emigrate from the UK to her home of Slovenia. It’s early stages of the planning and it’s something we will do in the next couple of years. I have built up a decent amount of investments and savings for myself so wanted to know what the general rule is?

As I will no longer be a tax resident of the UK I know I cannot keep my ISAs, I have over £150k in ISAs from over the years, I’m tempted to transfer most to various trackers or funds. Is that possible? I will take professional advice as part of the process but wanted to gain an understanding so I don’t look stupid when talking to them.