Maybe just like the US, Britain got complacent about their past history and stopped investing in infrastructure, stopped trying to be a leader in science and their youth got pampered/soft while other less developed countries hungrier for growth started becoming more competitive by investing more in infrastructure, spending more on science research as a % of GDP and their youth are more motivated to succeed in life.
The UK is
very different to the US. Completely different culture.
The American Dream is all built on an individual succeeding and something that's to be celebrated.
Success in Britain might be someone stepping out of their class lane, triggering the "who does he think he is?" effect. It's lessened considerably since the days of David Beckham's sending off, that made a lot of people very happy to have a legitimate reason to have a go at him instead of just crab bucketing, but still, it's still floating around in British society and the people running the current government are exactly the kind of inbred upper class c*nts who're appalled by the idea of social mobility and really don't think poor people should have the time to play sport when cheap manufacturing is happening in China that could be happening in Britain - 12 year olds with no prospects should already be in the workhouses making a profit for the aristocracy, wot wot, old mucker! Those people don't want to see money invested in sport that could and should be in their own bank accounts (preferably overseas where they don't have to pay tax that might make the poor people in their own country that they want to exploit have slightly less miserable lives... by the way, these people love "patriotism" because it means poor people doing things against their own interests... it's incredible that that **** still works but apparently it does).
Anyway, the infrastructure for sports such as football*, in particular, and rugby is good even though engagement with those two sports fairly consistently splits along cultural lines. There's lottery money that's been well used to invest in sports that feature at the Olympics but it's rarely waved around at well established sports. Tennis seems to be caught in between because Wimbledon is huge but tennis, in general, isn't (though Murray's success seems to have had a positive impact on youth engagement and coaching on the basis of how much the standard of league tennis has risen in the last 4 years).
The infrastructure for tennis is mixed. There are loads of well established clubs around, almost all on all weather surfaces now, but it's not like the LTA invests much, if anything, into them as far as I'm aware. To get indoor facilities, which Britain absolutely needs given its unreliable weather, you need money invested to build facilities - that's simply not coming out of local council coffers for municipal indoor courts and the only business doing it is David Lloyd Leisure and if you want to compete for a court at one of those you have to be a paid up member, which might cost you 10x more per year than just joining a local club because you
have to pay through the nose for gym facilities you might never use, nor ever want to use.
What probably should happen is some sort of investment into overhead cover for the courts somehow, without limiting how high a ball can go too much, but that needs money that the local clubs probably don't have (I dunno about more affluent areas of the country but they have to look for outside investment to raise funds for a new clubhouse in my area...) and even if they have it the chances of getting it through planning permission without an outcry from the local NIMBYs is essentially zero. Hell, even getting an indoor centre built anywhere new will have locals losing their **** as it'll probably be on land they can't possibly countenance not being a field.
* as for British (well, English) football... there was a cultural identity problem that came along with Dave Bassett up and under football that morphed into the Allardyce/Pulis "pragmatic" football that dovetailed with clubs having the money to keep their pitches in good order and being able to pass the ball along the ground successfully became a thing, so whilst Barcelona was readying the carousel the so called "golden generation" of players was full of Roy Of The Rovers "heros" that either tried to pull off Hollywood moments with the ball or ran away from it lest they get stuck with nowhere to go and no-one to pass to because players from rival teams didn't want to help them out. Things have improved on that score significantly over the last 5-10 years.