I use GiffGaff, along with the OH and a few guys in the office. Can’t speak highly enough of the service, definitely recommended.
There isn’t a way of automatically buying goodybags, but the mobile site is very easy to use so you can do it via mobile. You can also queue goodybags up if you aren’t going to be able to top up for a while.
You can set it up to automatically add credit to your account – for use on numbers that aren’t included in the goody bag/for when you run out of minutes – this seems to work well, they might add an auto-goodybag function at some point.
Definitely recommend giving them a go.