4G speeds atrocious no phone signal either..

I had a similar problem with very poor ACTUAL coverage/signal despite Ofcom, Smarty, Three all reporting good 4G coverage and no reported problems.  It happens - sadly.  Often with mast upgrades that are planned + not yet complete but as such are NOT required to be shown in the "problem/outage" database despite, clearly, causing a problem/outage.  [I learnt this eventually from the Smarty Customer Support team.  They admitted the problem, refunded my monthly charge immediately[.

Trial and error.  Try Giffgaff (MVNO and sub of 02) or Voxi (MVNO and sub of Vodafone) or Lebara (MVNO on Vodafone) or Lyca Mobile (MVNO on EE) instead and see which one actually works for you on the cheapest tariff before PACing across.  [Here a dual SIM phone helps with running tests].   Not as cheap perhaps as Smarty but different networks so different coverage and some have better international roaming.