This is not true, the guidance is:
"one form of exercise a day, for example a run, walk, or cycle - alone or with members of your household"
There is no time limit on how long you can be out exercising for.
This is not entirely true, not all agencies teach a Controlled Emergency Swimming Ascent (CESA). From memory the only one that teaches this is PADI.
Most don't as this is not a very effective or safe way of getting up once you get bellow a few M of water.
Most agency's will teach you to do an Alternate Source assent where you come up breathing of your buddy's air source, as this allows you to come up at a safer and more controlled rate.
Id say it comes down to the way cave diving in Briton is largely different from that of mainland Europe. So even on site the British cave divers would see the approach as different to the Europeans
In Briton cave diving is largely done to move between flooded sections of dry cave, so most cave divers are dry cavers first, and diving is just used to bypass sumps.
Also most British cave diving is done solo (so without a buddy that most systems recommend.) in quite tight conditions.
Caves in mainland Europe tend to be larger and the diving philosophy is different. There you will find team diving prevalent.