Dear Andy,
Welcome to Pprune.
Microlight schools can be found at North Weald - flying with Jet Provosts etc; or at Sywell, one of the biggest and busiest GA airfield in the UK,; or, closer to home, at Perth, where there is commercial flight training going on as well as it being Scotland's biggest GA field.
In fact, microlights can be found at almost every type of airfield in the UK that you will find GA, with the exception of major commercial airports such as Glasgow, Edinburgh, Newcastle, Manchester etc. And usually for the same reason you don't get GA at Heathrow -because of capacity issues and cost.
You can also find GA types flying from fields - such as the farmer on Collonsay who can't park his Cessna 172 on the island's airfield! - or many LAA homebuilt types.
Most of us want our flying - whatever type (jets, twins, SEPs or microlights) - to be affordable. So if you only are allowed to fly day VFR, why pay the cash to operate from an expensive radar-served airport?
Nothing worng with operating out of a farmer's field - and of course just because you have taken off from one doesn't mean that is the only type you can fly to!