a) what country are you from?
b) what do you look like?
I put my first-world passport1 on the counter and I wear a dress shirt when I get off the plane. Haven't been asked once about money in any form, itinerary, employment or really anything else other than the standard "where are you going", "how long" and "anything to declare" questions. Even the newly-hired girl going through the standard question list in order didn't ask about money.
If you look like a backpacker and come from a place that has a high volume of people trying to work illegally in the UK you can probably expect an interview room. £200 in cash, detailed itinerary, bank statements showing another £2000 available and a credit card with a good balance will go a long way to supporting your status as a short-stay visitor. Don't bring too much cash - only the foolish carry huge amounts of cash around London, Immigration knows this.
- I will expand this a bit by adding that the UK routinely approves work and immigration visas from my country, so I have zero incentive to work illegally.