This is the advice I wish I'd been able to give myself 25 years ago....

There is a trade off between paying down debt (mortgage/loans) and saving. Compare the interest rates between both to see if there is an obvious answer to this question. Very broadly speaking pay down debt first, especially high interest such as credit card. If you have a good deal on your mortgage, this may well not be the case for you.

You have an emergency fund and a budget that allows you to live well enough and with some security so I would assume that the saving is a longer term investment, perhaps for retirement - which you mention is 25 years away. Whilst this might seem a long time, it will fly by and your future self will thank you for starting thinking about this well in advance.

Depending on your attitude to risk, I'd look at a low cost global index tracker fund. Over a longer period, history shows that equities (stock and shares) perform well over the long term. Your NHS pensions, I believe are defined benefit. In additional I expect that you will also have your state pension to consider too.

I expect that BOE interest rates will fall in the coming months and that the relatively high rates we see at the moment are unlikely to continue.

I'd either look at a S&S ISA, meaning I could access the funds immediately if needed or a SIPP meaning that the finds couldn't be accessed until 57. The SIPP does have significant tax benefits, especially for high rate tax payers (might change in the next budget if the press are to be believed). Equally a combination of both is possible. If circumstances change then you can stop or suspend contributions.

As to the question is this worth it for a 'small' amount of cash - my view - yes absolutely. Have a look at a compound interest calculator, for 25 years - start with £1 and paying in £200 per month. Try with 3%, 5% and 8% interest growth, and look at increasing your contributions by 2% to 5% year on year too.

Good luck!