While it was not UK, this might still be relevant.
While I was traveling to Germany on my new passport for the first time, I was detained for several minutes when I couldn't reproduce my return flight. I was then asked how much money I have on me. It was if I recall correctly €400. I was also asked to produce the money so the border police can verify it. I was even asked to spread the bills so that he can count, he said he is not allowed to touch them. I know this was a cascade of mistakes on my part, I should be carrying my old passport along with a printed return ticket. If I hadn't done these mistakes, my cash wouldn't even be asked.
As long as you have a support letter or reservations and a return ticket, I don't think anyone would ask you how much cash you have on you. If you state that you have £25 without being asked, you will either be laughed at or will be assumed to act suspiciously.
In several circumstances that I entered UK, I was asked to translate for some Turkish visitors (although I am not a sworn translator). In none of those occasions, I ever heard of the amount of cash the visitors have on their person.
I hope this helps.