I already have CoH and Opposing Fronts on disc. If I get Tales of Valour through Steam, am I right in thinking I can just run Tales of Valour and it'll detect that I have all of the games, or should I also get the Gold pack for the extra £3.74?
Yup, it's all linked to your Relic account. You don't even need to download/install ToV if you've got it installed already. Just click the "CD Key" link to the right of the Details view, and then in CoH, go to your account page, and register a product.
Of course, the other option is to download it via Steam, and then you don't ever need to worry about your discs again - and you'll be able to track the amount of time you play it, too

I already had CoH, but not OF or ToV, but for the sake of a couple of quid, I purchased them all on Steam again, but I realised now I didn't need to, just the ones I didn't have.