Hi Powerfull. I just moved here from London 6 months ago.
To get an exact figure on PAYE use this calculator:
Take home on £50,000 is £2886 per month
Take home on £40,000 is £2395 per month
1. Cost of Living. For sure everything is cheaper here in Munich from beer to clothes to accommodation. I think 1.5 times for expensive sounds about right - depends what you spend your money on of course!
2. Apartments? There was a thread recently citing London as 2nd only to Tokyo on costs. Central London could be up to £2,000 a month for a nice apartment but most people share of course. A "2 room" over here is called a "1 bed" (1bed + 1 living room) in the UK! If you move out to the suburbs (see below) then budget £600-£700 a month for reasonable 1 bed place. You can get 2 bed for £900 which is why people share...but rentals are certainly down.
3. I am biased. I dont think you would need to go truly CENTRAL to get lively bars and restaurants. West London is the only direction to be. Putney/Clapham is a short tube journey to London but rent still pricey. Depending on where you work (e.g. is it close to Waterloo station?) then you could go for the London edge of Surrey/Middlesex such as Twickenham/Richmond/Teddington (my patch!). Plenty of place to run in Richmond Park etc.
Because London is so big compared to Munich (12 Million as opposed to 1.3Million) you almost have cities within cities. You will probably never step foot in East London for example (probably best actually).
£40,000 to £50,000 is enough for you to survive fine in London but you won't be living like a king!
I used to work in the City as a foreign exchange broker in my 20's and PERSONALLY hated the whole frantic/busy/tourists/traffic/smoke/bars scene and couldnt wait to get out to clean air and less stress of the suburbs. Covent Garden etc for me is full of pretentious ****ers. That was 15 years ago, I was earning £mucho per annum, with company car, unlimited expenses and still had a huge overdraft!!!
So in a nutshell - go for it if you can commute from Richmond/Teddington area to work and then you can have the best of both worlds - but the full-on Kensington scene will be a culture shock compared to Munich.