throw923498
In zones 1-2 (or 3) that'll probably have to be a houseshare - but you'd still easily get a nice private room for that. The takehome of £45k is £2,797.27 /month after tax[0]. Even if you spend 50% of that on rent (inadvisable but common in London), that's still £1400 a month to play with. Which is still more than the total takehome of someone earning the London living wage[1].
I mean, you'll have to have a commute if you want a private, 1 bed flat or larger and you're working in Zone 1, but that's not exactly much of a hardship. My point is that with even the most basic level of sane budgeting, £45k is easily enough to live/work pretty much anywhere in London.
[0] http://www.thesalarycalculator.co.uk/salary.php [1] http://www.livingwage.org.uk/what-living-wage
I would call 3 years commercial experience below average to average. Depending on ability it would be high-level junior developer to low-level mid developer. £40-45k would definitely be the high end of what you could expect, and it would rely on you interviewing very well to get it.
PHP is not a technology stack that's hard to find people in, and therefore salaries tend to be lower. Other tech stacks that are more specialised tend to attract higher salaries, but there are less positions available.