Hi Wilson c
1) You are right - while you can't contribute to your ISA after you leave, you can keep your ISA open, and you’ll still get UK tax relief on money and investments held in it.
2) You may not have to declare your savings interest, as it may qualify as 'disregarded income'
Please see guidance here:
SAIM1170 - Savings and investment income: non-residents
To allow us to correctly advise you here, we may need more information
Please contact us by webchat or phone here:
Income Tax: general enquiries
Thank you.