Upvote:3
According to the letter of the published rules, unless exempt from various parts of the rules:
The last page gives instructions for those on short trips:
If youβre travelling to England for less than 10 days, you will need to quarantine for the whole of your stay. You must travel directly to your place of quarantine when you arrive in England and directly from your place of quarantine to the port or airport when you leave. You should follow safer travel guidance and avoid public transport if possible.
As to the why, it's cheaper (for the UK government, not you) and easier to enforce if all arrivals follow the same rules, unless there's an economic reason not to. In particular, that makes it harder for a change of plans to mean people don't have a test they've already paid for.