Thanks for all the advice everyone. I got an imperial feeler guage (took a week with express delivery from amazon) and re-measured. I got 3 thou for the liners, but not more.
So I decided to order a brand new head with valves and I'll sell the old one after taking the cams out, not worth the risk of refitting a soft head.
Also got the ultimate MLS kit with the uprated oil rail from DMGRS
I know there's some controversy saying that the oil rail is only needed for Freelanders and not MGs but I've also heard that MGR advised always fitting it, so why not.
I'll update when its all back together, hopefully it will all be running ok!
Edit: I decided not to go for the modusengineservices.co.uk modification as mine has already been skimmed at least once and the valves look pretty dodgey too.