If it's one of the bigger ISPs then they're obligated to provide you with a minimum speed guarantee and if it drops below the guarantee and they can't fix it they have to release you from your contract.
Get a guaranteed speed today when you shop around for a new broadband deal.
www.ofcom.org.uk
The fact that other ISPs have declined to supply you in the past though means they know your line/local infrastructure is poor and don't want to be bound by the speed guarantee, so changing ISP is unlikely to improve your situation much unfortunately
The other option is, BT (& KComm in Hull) are bound by Ofcom's USO to provide everyone with a minimum internet connection of 10Mbps down, 1 Mbps up.
The only downside being they're only obligated to spend £3.4k on infrastructure to do so, anymore than that and you'd have to pay the excess if you wanted to go down this route.
Everything you need to know about your right to request a decent broadband service.
www.ofcom.org.uk
Though I know they have provided 4/5G and satellite solutions to deliver their USOs under the £3.4k limit in some cases.
As you're in Wales it's probably worth investigating what, if any solutions/funding the Welsh government can provide.