My nurse practitioner doesn't consider me to be in the 'vulnerable' group and I should exercise as normal. So from that point of view the century could be back on; I'm in two mind whether or not to do it though
Who qualifies for membership of the 12 week house arrest group does appear to be open to question.
As someone who is properly ill, stuff like exercise and getting back to work are recognised as aids to your recovery.
Perhaps, at a stretch, you could justify a century ride as a medical need.
The advice is just that, advice, but if you simply ignore it you are still subject to the same restrictions as the rest of us in terms of one hour of exercise and so on.
Here's a link to the 12 week restrictions.
Quoting: "Shielding is for your personal protection. It is your choice to decide whether to follow the measures we advise.."
https://www.gov.uk/government/publi...ng-extremely-vulnerable-persons-from-covid-19