It might be useful to you to find a forum of UK keepers just to see where they buy their equipment. It seems that wooden vivs are more common there than in the US, as well as the bulb-in-cage setup you describe. There might also be brands of heat mats that are designed to be used inside a viv that aren't sold outside the UK for all I know.
Incidentally, a bulb in a cage that is entirely inside the wooden viv isn't going to be nearly as bad for humidity as American keepers assume it is. The reason is that in the US the more common way to use a CHE or a heat lamp is to put it in a fixture that sits above a screen top of a tank, or above a screened-over cutout in the top of a vivarium. So the freshly heated air rises right out of a big hole in the top and takes the humidity with it. If your entire fixture is inside the viv, you won't have nearly as much loss of humidity (and heat, for that matter).
As for whether the 75 watt bulb you have is adequate, you'll just have to try it and find out what temperatures you get. If your house is 50 degrees it's probably not enough, if your house is 75 degrees it's probably way more wattage than you need. But as long as the wattage is at least adequate, your thermostat should take care of the rest.
BTW, healthy ball pythons do not need calcium or other vitamin supplements. They get everything they need from their whole prey. The only reason you would ever need to supplement would be in case of illness, and that would be only at the direction of a vet.
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