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ISA - intra year profits and switching process
An ISA is a much simpler thing than I suspect you think it is. It is a wrapper or envelope, and the point of it is that HMRC does not care what happens inside the envelope, or even about extractions of funds from the envelope; they only care about insertions of funds into the envelope. It is these insertions that are limited to £15k in a tax year; what happens to the funds once they're inside the envelope is your own business.  Some diagrams: Initial investment of £10k. This is an insertion into the envelope and so counts against your £15k/tax year limit.                 +---------ISA-------+ ----- £10k --------->              |                +-------------------+  So now you have this:                 +---------ISA-------+                |  £10k of cash     |                +-------------------+  Buy fund:                 +---------ISA-------+                |    £10k of ABC    |                +-------------------+  Fund appreciates. This happens inside the envelope; HMRC don't care:                 +---------ISA-------+                |    £12k of ABC    |                +-------------------+  Sell fund. This happens inside the envelope; HMRC don't care:                 +---------ISA-------+                |    £12k of cash   |                +-------------------+  Buy another fund. This happens inside the envelope; HMRC don't care:                 +---------ISA-----------------+                | £10k of JKL & £2k of cash   |                +-----------------------------+  Fund appreciates. This happens inside the envelope; HMRC don't care:                 +---------ISA-----------------+                | £11k of JKL & £2k of cash   |                +-----------------------------+  Sell fund. This happens inside the envelope; HMRC don't care:                 +---------ISA-------+                |    £13k of cash   |                +-------------------+  Withdraw funds. This is an extraction from the envelope; HMRC don't care.                 +---------ISA-------+ <---- £13k ---------               |                +-------------------+  No capital gains liability, you don't even have to put this on your tax return (if applicable) - your £10k became £13k inside an ISA envelope, so HMRC don't care. Note however that for the rest of that tax year, the most you can insert into an ISA would now be £5k:                 +---------ISA-------+ ----- £5k --------->               |                +-------------------+  even though the ISA is empty. This is because the limit is to the total inserted during the year.