You'll get nicked if you do. They have an Ironman race here starting in Weymouth and heading through Dorchester towards Sherborne. Its an absolute pain in the arse. Last year they published no warnings, just shut every single approach road to Dorchester from every direction. There were no detour signs, people were simply being stopped and told to turn back. Police bikes were patrolling the race route and nicking anyone who tried to ride through (on empty roads).
There were massive traffic jams up every back lane as cars tried to find a way through and then got stuck, unable to turn round. Some people ran out of petrol because they couldn't get through to Dorchester and slip roads which would have allowed them to get on the dual carriageway to Bere Regis were closed as well. It was utter chaos and they're doing it again this year.
Here's the info. Avoid like the plague.
https://www.dorsetforyou.gov.uk/travel-dorset/ironman" onclick="window.open(this.href);return false;
If you own a horse, please ride it along the Ironman route on Sept 17 and encourage it to shit as much as possible.