the fact my current pension pot is already growing by more than £40k+ a year so even if bought as a stock & share ISA pension product I think I would be liable for a tax bill.
Pensions and ISAs are entirely separate, so what you do with one doesn't affect what you do with the other (apart from running out of cash to invest!).
Both offer a saving on the taxes you would otherwise pay, and either (or both) are potentially suitable for saving for retirement. Both have restrictions making them less suitable for short-term investing. And both allow you to invest in the same kinds of things - the pension or ISA is often described as a "wrapper" around the underlying investment, giving you tax advantages in exchange for constraints on what you can do.
Pensions are extremely complex and the rules keep changing, but broadly the tax proposition is that you pay in from untaxed income, don't pay any tax while you hold it, but do then pay income tax (hopefully at a lower rate) when you draw money out later. Lots of restrictions on exactly when and how much you can put in or out.
ISAs have remained remarkably simple since they started in the '90s (originally called PEPs with effectively the same rules). There is no tax saving on the way in: you use income on which you have already paid tax (or other unrestricted cash you have lying around). However, you then pay no tax while you hold the ISA, nor when you take the money out. The snags are a hard annual limit on what you can put in (currently £20K per year, was lower in past years), on a use-it-or-lose-it basis, and also once you have taken money out you can't put it back (other than as part of the current year's max £20K going in). You can move your accumulated funds from one ISA to another without losing the tax protection, but as soon as you've taken it outside the ISA wrapper then the opportunity to grow further untaxed is gone.
Pensions are so complex you need professional advice to even start thinking about them but the advantages can be great (or not). ISAs are simple to understand, but the benefit is only large if you are able to put in every year and build up a large fund without ever being tempted to pull it out again. Given both have input limits, for people with large amounts to put in then "both" has to be the answer. If it's a case of one or the other, professional advice is required to choose (and even the professionals are only guessing because there's so many factors that could change in the future).
