I think you're mistaken if you got the impression that the museum once had guides. This isn't a recent trend, so far as I know it's been delightfully free from tour guides since 1881.
replyYou had to buy tickets prior to 2001, so that's changed. (Was entry free in its early history too? Not sure.) That used to be your greeting, the ticket desk.
They had an earthquake machine in 1985, it must be the same one.
The NHM is free to enter, some special exhibitions charge for entry and I think some require free booking to manage crowds. There is a very strong encouragement to make a donation though.
replyAs a tangent, I find it a bit annoying that so many UK museums advertise free pretty aggressively and then provide such "very strong encouragement" as you put it to attend. Mind you, there's less direct pressure than there is in some places. The Met in NYC used to have an optional but not really optional policy for museum admission as you got your pin though it now not optional at all for non-NYC residents.
replyFree to enter since 2001. Which means now they have (more) donation boxes.
replyMy local museum started charging for entry a few years ago, along with a refurbishment, new exhibits, a bigger gift shop and a push to attract more tourists. So now it's horrible. I'm not sure what the unifying mistake is in both models, free entry and ticketed. I think the error might be in trying to serve the public.