They vary wildly across Britain. I give some examples:
50 Free Girls age 13
Middlesex = 32:50
Essex = 35:20
Devon = 34:35 (includes Plymouth Leander, Mount Kelly)
Derbys = 37.20 (2016)
Staffs = 32:00
Kent = 31:00
Hamps = 31:20
Berks = 32:40
Yorks = 32:80
Middlesex includes parts of West London whilst Essex spreads towards the East. Middlesex includes clubs like Barnet, Westminster & Chelsea from where a few participants in the British nationals sprung.
Out of those areas listed above Plymouth Leander (Devon) and Sheffield (Yorkshire) supply more swimmers at national level than the other counties and yet they have slower county qualifying times in the 50 freestyle for example than the faster Middlesex and Staffordshire.
I have witnessed it myself, if the county times are too fast for at least 2 swimmers per club to qualify, the local clubs loose momentum and turn into leisure swim clubs and don’t become more competitive. Clubs need to be able to connect to the competitive events.
Hence even at the Olympics they now allow the fastest swimmers from developing countries to swim in the heats, even if that time is slow compared to our standards to bridge a connection.
From what I observed, talented swimmers can be very fast aged 9/10 if they start swimming early enough but then when swimmers turn 12 upwards it becomes a matter of regular training and eventual muscle-building, which is only allowed in British gyms from age 13. I think all County Associations should put on times that allow a wide participation even if it means that they struggle to engage enough officials to cover the events.
See the lists of all teams in the 2 divisions here.