astro turf would be amazing... not exactly cheap, but would be ace.
Cheap £20 to £35 box's of paint, go through about 1-2 box's tops a day.
I know the higer division teams always have their own paint, alot of people area buying paint en-mass and storing it in their own homes to save some cash, you'll have to test out how much paint you'd use.
There are alot of fields around London area, if you did want to open one u'll have to get alot of support(I say this now because i didn't see where your location was previoulsy) I know in the midlands there ain't that many fields
one of the reasons i decided to move away