But this is WW1 we're talking about .....
The tank crews would have been relatively indistinguishable from the PBI. Firstly because they were worried they'd be shot as spies if captured (tank crews were at some risk of that anyway), and secondly because, if the tank was knocked out, the survivors were expected to demount the Lewis guns and fight with the infantry. So conformity was key, and comfort and convenience would have been very low on the scale of importance.
Officers wore 'Regimentals' - tunic, shirt and tie. Below the waist was a bit more optional - overalls, riding breeches, trousers, even shorts. It was all about status, and demonstrating responsibility - not wearing a shirt and tie was probably a Courts Martial offence

. By the end of 1917 most 'Regimentals' would have been Tank Corps or MGC, but some officers would have retained the tunic and badges of the Regiment they were originally commissioned into.
OR's would have worn their tunics too, despite the heat inside the tank. Underneath they would have been likely to wear brownish overalls, but it was by no means universal.
Headgear would have been a mixture of leather helmets, 'Brodie' tin helmets, and caps with the stiffeners removed (known as 'cor blimeys' due to the normal NCO response !), with the peak at the back so as to not obscure vision on the gunsight or vision slit.
All ranks would have worn Regimental badges, rank insignia, and the white Tank Badge which denoted completion of training. Sam Browne's would have been common, as sidearms were standard. Gas mask cases would also have been likely. Given the speed needed to evacuate a burning tank (it's been estimated a man had about 4 seconds to get out

) all equipment would have been worn and not 'hung up'.
So, in summary, the 'John Colman' figure gives you everything you need for an officer, although you may need to amend the badges. I think Dragon do (did) a Lewis Gunner which would serve for Enlisted Men. Overalls and headgear to taste.
Incidentally, the Germans approached it in the same way, ie. largely using standard infantry clothing, but with leather patches on knees and elbows. Officers tended to wear leather jackets, some privately purchased (for
Liesel's crew, I kit-bashed from the 'Lutz Fedder' figure with a 'Century 21' leather jacket). The French, of course, deviated from the 'norm' issuing leather coats ....
Hope this helps,
Chris