This is an extremely personal question and involves many factors, some of which have already been mentioned.
You should have some idea what it costs to live in the UK, as you have inferred that you already live here, and only you know what your own personal cost of living is.
The more cynical of us would think that there is an ulterior motive behind your questioning but in answer to your question the answer is, 'as much as you are able to command'.
I live in one of the more expensive parts of the UK and can afford to fly one or two hrs a week on much less than the figure already suggested.
As you obviously have access to the Internet I suggest you do a search for flight schools in your area and look at their rental and instructional rates, then you will be able to make up your own mind.