I've done one SMLE, a III*.
I did it without a proper action wrench, using a large padded adjustable, in a home made barrel vise, with a slight amount of heat from a propane torch.
It was tight, but, not exceptionally so, and with the heat was no big problem. Just take it easy, and go slow, with lots of penetrating oil.
Headspacing the replacement barrel was accomplished through a rather crude filing operation. Worked out ok though.