To try things out, you can always get a PAYG SIM from Three (or Smarty or IDMobile) before committing yourself to something long term.

I prefer Three over Smarty/iDMobile as they are the only network where you can get a public IP address, as I am running VoIP over 4G and did not want the problems associated with CGNAT. Alternatively, you could get a public IP through a VPN provider and use Smarty or IDMobile if that works out cheaper. Just for speed tests etc. it should not matter who you chose and speeds should be pretty much the same on Three, Smarty or IDMobile.

My router connects to the same mast on Band 3 all the time, I am probably lucky as we are in a semi-rural location and the mast is probably not too busy most of the time. But in more built-up areas 9with more users) there should be more capacity deployed and you should see reasonable speeds as well.

you dont quote how much data you get for that amount