Is Hive not tied to British Gas? Read that some of these are tied to the supplier.
Hive is a BG product, but not tied to BG.
I have a Nest - very nice, does what it says on the tin (as does Hive - my parents have that).
Honeywell Evohome is great but at over a grand to do this house I couldn't justify it - and, call me sceptical, but it's "technology" from a company more used to (I am doing them an injustice here
) bimetallic strips, so you know it'll be obsolete in a year and have all the security of a wet sock... whereas at under £200, I can justify binning the Nest in a few years when it's outdated, compromised and obsolete.
(I've also heard that the electronic TRVs can be rather loud and unsuitable for use in bedrooms, but YMMV on that)