Crazy, Good luck
Not sure if you will suffer it in the UK. but in Australia the system does not flip between 3G and 4G automatically. Even when the phone or device settings say prefer 4G or Prefer 3G it will not go between 3 & 4 G networks Automatically. What happens instead you get pushed or parked as something like 24kb/ps while other stronger signals receive commanding access over yours.
Whats more you probably will be stuck with old Android o/s that give 4G/3G/2G or 3G only options without 4G/3G options.
So you will have to go in ans flip the settings to see if 3G is less busy than 4G at the moment.