I'm surprised no one has mentioned this yet: you mentioned that you do not know what pressure you've been inflating your tires to, and that you are doing so by feel. There are a lot of possible reasons you've had many wheel failures, but this is certainly one of them. At the bare minimum you should get a tire pump with a pressure gauge. There is a lot of information on recommended tire pressures. http://www.bikequarterly.com/images/TireDrop.pdf is a frequently recommended method.

At your weight (11 stone is 154lbs or 70kg) you shouldn't need especially strong wheels because of your weight, although your riding style and terrain certainly matter. Spokes break when they have been detensioned during riding by excessive loading, which can be from excessive weight (probably not your cause) or transient shocks (in part maybe by insufficiently inflated tires, and riding heavy in the saddle over bumps). Properly built wheels appropriate for their application should not exhibit the problems you're experiencing, and should only require rare and minor servicing except in the case of unusual stress (crashes).

Hubs have changed since the manufacture of your bike, with a transition to freehubs from freewheels and to increasingly wide locknut widths (120mm probably for your bike, 126mm for 7 speed bikes, 130 for 8-11sp road bikes, 135 for 8-10sp mountain and some touring/cross bikes). Wheels compatible with your frame/drivetrain are a little harder to find in stock, and may be of lower quality. You will find this problem largely solved if you get some genuinely good wheels, possibly hand built from parts by a skilled local wheelbuilder, or by a reputable online wheel manufacturer. Also, on a steel frame like yours, a shop can cold set the rear spacing of your frames to be compatible with modern wheels, although it might require the replacement of some of your drivetrain.

For learning wheel truing/building skills, a deeply messed up wheel like yours is a rough place to start. It's a lot easier to make minor truing adjustments to a essentially functional wheel, or to even build a wheel from scratch from unabused components. Don't give up on learning, bikes really aren't that crazy to work on all things considered.