It various massively by area - how you expect prices to rise or fall for the area you're interested in, plus the rent cost vs purchase cost.
With those figures I can't imagine someone would be better off renting. But £25k is an unusually high rent (relative to national norms). London's behaviour has been different this year to the rest of the country too
The calculation is pretty simple in my mind (at least ignoring compound effects as typing on a phone!).
Take the value of the house (based on recent sold prices), e.g. 400k and predict house price chances, e.g. down 1.5% next year. That gives you a figure of 6k.
Next look at the opportunity cost of buying, I.e. what would that 400k be earning elsewhere. My investments earn an average of ~3% net at the moment so that gives me a figure of 12k. (you might say that not many people can buy a 400k house outright and therefore have that money to invest elsewhere, but you have a similar calc to do for mortgage interest if that's the case)
Next the cost of owning vs renting. Owning carries a maintenance cost plus potentially service charges, say a total of 2k a year (this is low when you bear in mind that this needs to cover the ' big' infrequent costs too).
So...6k + 12k + 2k = 20k. If the rent was over 20k then you'd be better off buying.
Except it's slightly more complex as we have to add the one off fees for buying and selling. So that 400k house has cost say another 18k in solicitor fees, stamp duty and ea sale fees. If you work on a 6 year period then that's 3k a year meaning that the ' break even' point for rent vs buy jumps to 23k.
Bored yet? :)
Of course my decisions are based on my area, local price changes, local rents, what I achieve on other investments etc. everyone's own set of calculations will be different, but the important thing is to consciously think about it rather than simply follow the ' renting is dead money' manta.