I wouldn't suggest a first time rehairer tackle a Show and Glow. While rehairing is not hard, it does take some experience and best done on ponies with a softer and more forgiving plastic. I learned that myself the hard way when I rehaired a Twinkler. The plastic of the sparkle ponies and the show and glows is very similar and it's very prone to cracking and spliting -as I experienced with Twinkler-. I would say that while it may save you money to do it yourself, it may be better to have a more experience hand tackle it.
If ever you want to start rehairing, I would highly suggest starting with the Princess Ponies as rehair candidates. I've only had a Moondust split one plug in her crown area, and that was because I was doing a -second- rehair on her and got too much on my rehair tool. I've done two other Moondusts besides her, I'm working on a Serena, I've done also a Taffeta and the plastic for some reason is just hard enough but still stretchy enough that it doesn't rip or crack. Sparkle and Glow and Shows are too hard, whereas a lot of the normal release ponies have very soft but too soft vinyl that it just rips if you put too much on the rehair tool.
I'd imagine a rehair done by an experience hand in the UK would run around 15-20quid, just as a guess. Most in the US charge around $20-$30 depending on the job and materials.