The RST will be ideal for that sort of use, good luck with it. We would all love to hear how you get on. With the bike already built an on offer, I guess you didn't get much choice re tyres etc., but that is nothing to worry about - SJSC will have fitted it with sound all-round tyres and they can easily be changed if you later fancy something different.
Enjoy the bike! I've done the TransPennine trail, with a few add-ons - from Lancaster to my home in Sheffield, with a deviation to Macclesfield to stay with my daughter. I was on a Raven Tour at that time, a bit heavier than the RST, I now ride a Sherpa with lightweight wheels, forks etc.
Lewis