While the OP is a bit confusing, if the facts are as I read them, he is not entitled to a refund.
1. A segment of a ticket was cancelled. At that time, his EC 261/2004 Section 8 rebooking options were a refund or an accommodation. He apparently took the latter, presumably because he wanted to travel.
2. Now, the circumstances have changed because OP will face a 14-day quarantine on arrival into the UK. But, the flight he accepted as a rebooking is still operating.
If this is the case, he is not entitled to a refund because BA has not cancelled the rebooking he accepted. BA's special pandemic waiver policy does permit OP to voluntarily cancel and accept a credit against future travel. What EC 261/2004 does not do is give the passenger multiple options. E.g., cannot elect/accept rebooking and then later change his mind to a refund.
This would explain the difference between OP's two experiences. OP --- if I have this wrong factually, please advise.