[UK] Using ISA/GIA to shift more value into my ISA — anyone else doing this?

Using ISA/GIA to shift more value into my ISA — anyone else doing this?

I’ve been thinking about ways to move more value into my ISA beyond just the annual £20k allowance, by leveraging my GIA and market volatility.

The idea is this:
- When a stock I hold in my ISA (e.g. VUAG) is high, I sell it (no tax) and buy the same or similar stock in my GIA (e.g. VUSA).
- Later, if the price drops, I sell in the GIA (possibly crystallising a small gain or CGT-loss), and buy back in the ISA at the lower price.
- Over time, this gradually increases how much of my portfolio sits inside the ISA, without adding fresh cash — just using price movements to “rotate” value into the tax shelter.

I’m aware of the HMRC 30-day share matching rule, so I’m thinking of doing GIA → ISA trades using similar but not identical ETFs (e.g. VUSA and VUAG, or IUSA and CSP1) to try and avoid triggering it.

Has anyone else tried this? Any pitfalls I’m missing, or ETF combos that work better for this kind of strategy? Would love to hear from others doing something similar — especially around the CGT implications or anything HMRC might frown upond.