XDA has an active forum at
https://forum.xda-developers.com/xiaomi-mi-10-ultra.
For the UK use Wondamobile as they ship to Manchester and then ship to you, so they pay any import duty, delivery in about a week from Hong Kong at a good price, other suppliers out there too.
One of the main issues in the hardware is that it has no band 20 radio. In the UK some operators change their use of band 20 from 3 to 4G, O2 being the main one that may affect you. 5G works a treat as does 4G on all other providers but with O2 and any providers that use O2 network, you won't get 4G from band 20 masts. To be honest, now that most people use 4G, the 3G throughput that it drops down to on these masts is dam good and I don't miss 4G at all. Still get Sky Q out and about via 3G. My second sim is EE and that is fine on 4G, so something to think about before buying.
The stock Chinese ROM has Google but one thing missing on this is location tracing and history. Gpay works fine on the stock ROM.
As Xiaomi encourage custom ROMs, you can unlock the bootloader. As a precaution, once you fully verify your account in mi cloud, and you start to unlock, it makes you wait 168 hours, one week, before allowing you to finish the process. Once unlocked, you can flash the Xiaomi.EU custom ROM if you want, details on the XDA forum or the Xiaomi.EU forum. Paranoid Android may be providing one too at some point. The EU version has done an excellent job on this but it does still have some work to do on CTS profile security but it does have Widevine L1 ( the stock has all security). Gpay too has a temp bug hopefully being fixed soon to but it's ok on Chinese stock and as the custom weekly or stable branches of ROM are based on the Chinese updates.
Have a read before committing. I moved over after being Huawei for a few years but with them locking the bootloader and the Google debako by USA has made me swap brands and to be honest, I am quite happy to do so, Xiaomi are making some dam good customisable phones.