Bad news for those planning to activate Windows 11 with a Windows 7 or 8/8.1 key: users noticed that the latest Windows 11 preview builds no longer allow activation with old license keys.
Go to Amazon UK, bunch of really cheap ones there. You'll get the key via email instantly. Ideally, install a fresh windows 10 and immediately upgrade to 11, then the key will be upgraded and remains valid even if the original one gets disabled.
KeysOff has been very reliable for me in the past (I'm not affiliated with them, just a happy customer) but I'm pretty sure there are multiple VLK resellers out there for Windows keys at this point.