I think CD has hit the nail on the head when he says how fragmented the sport is. ,

Currently most regular tennis clubs welcome juniors with open arms.  There are still a few who don't but the membership is changing.  Most young single adults who join a sports club have no issue with kids as long as they are well behaved and obey club rules.  Most adults with children would not join a club where their children were not welcome, a sports club being a place where families can spend precious time together.  Those which maintain a 'kids 2nd if at all' attitude are starting to suffer membership issues as their current membership ages and new blood seeks a more inclusive atmosphere.  Most run beginner level squads and junior practice sessions, and welcome the older juniors into general social tennis and adult teams.  They usually also welcome non-member juniors into their beginner squads or make junior membership available very cheaply. However, any junior who shows promise or starts to do well competitively is normally encouraged to move to an HPC.  This makes it hard for the clubs to run junior teams and obviously prevents the juniors becoming further integrated into the adult club tennis.

The HPCs as I mentioned earlier in this thread, are only about performance tennis.  They are a place where juniors turn up, do their squads, fitness or whatever and then go home.  They are not somewhere where a young player can spend best part of a weekend socialising, practicing with all and sundry.  Most have a completely soulless atmosphere.  Some do enter teams in the premier (national) junior club league but they struggle to field teams.  Their players are out competing as individuals most weekends and, as there is no club atmosphere, feel insufficient loyalty to the HPC to re-arrange their schedules.  When they stop making progress at the HPC, very few juniors return to the club system.  As HPCs are about performance tennis and judge their juniors on results and measurable progress, those who aren't making the grade (most at the end of the day) leave in a state of disillusionment and negativity about the sport and generally turn to other sports or activities.

There are too many HPCs as well (soon to be changed) meaning performance players are very thinly spread.

Very few schools have tennis as a meaningful sport (Queenswood, Reeds, Talbot Heath and one or two others aside).  Not enough children can play it at the same time, and usually only 4 kids can make the team.  A lot of primary schools have adopted the LTA system for introducing kids to racquet sports but it doesn't really develop from there.

I can't help feeling that the solution lies in keeping the kids in club tennis for longer.  Matches/hitting against the top mens and womens team members is probably just as challenging as playing other strong juniors, at least up until 14 yo.  And those progressing more slowly will stay in the sport.  Maybe this is what the LTA are hoping will be the result of their planned drastic reduction in HPC numbers.