For small gas cylinders for camping stoves, the main question is the fitting on the top. They all contain a similar sort of fuel, usually a mixture of butane and propane. Some may have different proportions of these, but it doesn't make much difference, except in really cold conditions.
Probably the best and most common type is the resealable, screwthread canister. These are available from a variety of brands, eg Coleman, MSR, Jetboil, Primus etc. This is sometimes described as the EN417 standard. The stove just screws onto the top of the canister, and can be easily removed if necessary. These are available to buy in most outdoor shops around the UK, and should be easy enough to find along the West Highland Way, eg in convenience shops or at campsites etc.
There is also the older, piercable canisters. Most common is the Campingaz brand. These were more common, still pretty widely available, and usually cheaper than screwthread canisters. But the big disadvantage is they cannot be resealed. So you can't remove your stove from the canister, until the canister is completely empty. This is a lot less practical for hiking, if you want to pack your stove in your bag each day.
There are a few other types of fittings available. eg the Campingaz "Easi Clic" system. This is resealable, and the fitting looks similar to the screwthread fitting. But it doesn't have a screwthread, the stove just clicks on. So it is only compatible with Campingaz stoves, probably best avoided.