Sorry if this seems like I've gone off at a tangent but hear me out.......
Aldi have these vehicle trackers in stock next week for £40. You need to get a SIM for them to work, the provider and PAYG/contract, it's your choice.
External Link/Members OnlyIt seems to me that having one of these is a completely legitimate and suspicion-free reason for having an additional SIM. You can finance it in the open, no need to worry about keeping it secret. Anytime you fancy a punt, you just remove the SIM from the tracker and bung it in your phone (main phone, old phone, spare phone, punting phone, again it's your choice).
Basically, it's a way of having a direct debit of a few quid going out every month for an additional SIM for a completely innocuous and innocent reason. Yes I'm aware that there are still issues of anonymity to be aware of but my impression is that merely financing an additional SIM can be problematic for some chaps when they have a joint bank account with their other halves. Maybe this tracker thing solves that problem?