From personal experience I have a group of 3 in a 6ft by 2ft and a group of 5 in an 8ft by 2ft and they have runs 6ft by 4ft. The hutches are definitely not 3ft high though, more like 2ft. I can quite understand the RSPCA saying for a group of 5 you'd need something 6ft by 3ft though, the 5 I have in the 8ft by 2ft have enough space but more would be better. For the pair of boys I would have thought 5ft by 2ft would have been adequate. Obviously the minimum for 2 piggies is supposed to be 4ft by 2ft but with a pair of boys if you can give them the extra foot it's certainly worth doing.
I adopted a guinea pig from the RSPCA, just one neutered male and they were very happy that he was to live with just one female in a 6ft by 2ft. They came to visit for a home check and said everything was great. Maybe different branches have different rules but it does seem a bit odd.
You'd be pushed to find a hutch 6ft by 3ft, they just don't come in that size. Some specialist hutch companies make 6ft and 7ft hutches but they'd be 2ft wide I imagine and other than that you are looking at something custom built. My 8ft was made by a joiner to my specifications. My runs were from Ebay, £40 a piece but I had to go and collect them about an hour away! The 8ft hutch was a good price for the time and effort that went into building it but you'd be looking at £200-£300.
