Hi Beams for £20 delivered direct from Amazon. I've ordered multiple sets over the years and they are the real deal.
For DR/Ken Smith/Fodera/La Bella strings, I use Bass Specialties on eBay. The prices have gone up roughly £5 over the last four to five years but they're still cheaper than the UK. No import duty either since the seller knows what he's doing.
Regarding Rotosound, I'm a fan. Their customer service is second to none. I had a dead string on arrival once and they sent me not only a replacement but a whole new set. I really like the Roto Bass nickels and think they age very well. They cut through a mix and have a really good mid punch to them.