I’m with EE too and it’s been driving me batty. I have found that disabling VOLE, but keeping LTE enable makes it more reliable. Right now I’ve been going for almost 24 hours without losing my data connection, though I know for sure if I re-enable VOLTE I’ll lose data in under an hour.
I am in contact with Apple over this problem, they’re collecting debug data from my device so as to investigate the issue further. Hopefully there will be a resolution before too long, especially if more people supply debugging information to Apple engineers to investigate why it’s happening.