Well I can gladly announce that O2 4G800 is now showing on the coverage checker on the upgraded VF Higham Ferrers mast - so Higham Ferrers (and the surrounding areas) now have:
Vod 2G900, O2 2G900
Vod & O2 3G900 (from the upgraded Vod mast)
Vod 3G2100 (from the upgraded Vod mast) & O2 3G2100 (from possibly the old O2 mast?)
Vod & O2 4G800 from the upgraded Vod mast
Doubt that they have upgraded the O2 mast as it is lower down than Vodafone.
EE and 3 are still on 3G2100 - no 4G yet round here (EE 4G comes from the Wellingborough mast, very weak signal).
So at the moment, in my area, it's:
Best for Higham/Rushden/Chelveston: O2/Vodafone (they have 4G800 and 3G900, which should cover the poor spot in Chelveston)
Best for Raunds: EE/3 or O2 (the O2 mast is out of the village though, so EE/3 would probably be stronger in the town). The old nasty Vodafone mast is STILL 2G ONLY!!!
Best for East of Raunds (i.e. Thrapston, Oundle etc): EE/3. Vod/O2 still haven't bothered to upgrade that area.
Best for Wellingborough: All 4 operators have 4G and 3G, but I'd probably be inclined to say EE (in terms of speed). Managed to get 80Mbps+ on EE's 1800MHz 4G, whereas Vod/O2 are using 800MHz (so you'll be likely to get about 30-40Mbps). Haven't tested Three's 4G (it's only recently been added) but if their rollout elsewhere is anything to go by, it'll probably be beaten in speed by EE, and quite possibly even the CTIL duo. At least their 3G is fast enough.
Oh, and whilst on the subject of Three, I saw an advert about banishing blackspots (with their VoLTE @ 800). Trouble is, it only works if you've got a handset with Three firmware (so not an unlocked one bought from elsewhere) AND you can't get a sniff of 4G1800/3G2100! Pahahaha