I think what cybersoga means is why not drill the holes yourself. I run an install company, we would come to Sothamplton, but it's simply not economic if all you want is a wall mount fixing. The proper rate for that is say 40 mins work, make sure it's straight, in the right place, and test for cables and pipes before drilling. This is say, £35 worth of work. But, with an hour each way to get to you, and few will be closer, it's at least a morning, plus 100 miles of fuel. £250 is not unreasonable for that, then there is VAT. Go buy yourself a drill, £30 at B&Q, I think there's one near you? If you've never used one it's a life skill we should all have.
It's economic to do an install when there are cables to be buried, surround sound to set up, speakers to mount etc, a decent day's work for a decent price where you can see the value added. I hate people feeling ripped off for installation, there's just no way round it in your case, unless local handyman is prepared to do it. He might find the responsibility a bit overawing, and may not be insured though.
If it's a solid wall you have not a worry in the world. Drill at 10mm, get a plug to suit this, and use 3" scews, 8 of them in usual wall mount, with washers under the heads. It will hold the rock of Gibraltar. I usually do a couple of pull ups on the bracket, and if it falls off it won't hold the plasma.
Cruncher, you have blockwork, still not a problem, just like I said for you. Plasterboard, find the studs and screw to them, or fit a sheet of MDF to the studs ans screw the bracket to that. Lots of cable hiding options there too, try this thread for hints