Headspace checks in SMLE' are overrated, something Americans get all fixated on. You will never have headspace issues, the only issues you will get will be poor brass lifespan but that has more to do with SAAMI dies and chambers cut for rifles headed to war that needed to be reliable with any old ammunition one might have in a stinking hot jungle or sandy desert. Regardless, about the worst thing that will happen will be a case head separation which most people only figure out has occurred when the next round jams in the chamber, if your at a range normally someone has a case extractor from a bren gun tool kit
Rimmed cases are safe as houses.
You dont need to buy gauges, if you have an empty case, coke can to cut up and some verniers you can make your own in a few minutes.
British field gauge is .074, if it doesn't close on that dont worry.
Plenty of people shoot martini's with a lot looser than that and they dont blow up either