I believe the whole F35B STOVL concept for austere ops is flawed. F35B could not have gone to Kandahar in 2005 as the Harriers did, as its STOVL ops would have destroyed what little surface was actually usable - not to mention wrecking the airframes' stealth characteristics through FOD impacts. If bespoke reinforced concrete mixes are required for pads, what chance would MEXE or a third-world rough strip have? Even using rolling vertical landings would submit the surfaces to extreme treatment.
IMHO the UK's stance on F35B comes down to the following factors:
1) Choosing the same variant as the USMC, as they're the only US service fanatical about actually getting the F35. This increases our chances of eventually receiving some ourselves. The USAF would probably bin F35A for more F22 if they were pushed, and the USN would be happy with a load more F/A18 instead of being sole customers for the F35C (which has a different airframe to the A and B models).
2) Lots of lovely workshare and technology showcasing in the lift fan
3) Lots of Harrier pilots involved in the project who already know that STOVL is the answer, and offers a perfect chance to get bums-on-seats for their mates (having noticed with envy how the Jag force managed to 'own' the Typhoon stand-up).