"committed exclusively to British racing between March and October".
I think you will find that's restraint of trade & I doubt that a club will be able to show that it has been signed to protect confidential information, trade secrets, client relationships or goodwill. Therefore would be unenforceable.
Also do you seriously expect a rider to forgo higher value contracts in Poland, Sweden or even Denmark for 40 UK meetings? Say a top rider is on a guarantee of £3k a meeting thats £120k per year for a mechanic, three bikes, van, diesel, spares and servicing. I think most riders would turn that down.
You need to remember that is it no longer 1968, the world and even speedway has moved on