Welcome Tom & Lala.
I assume because you have posted the question on a Thorn forum, your meaning is 'which Thorn' model should I consider'....although some members do have non Thorn bikes as well.
If that is the case, then I guess the question would have to be, what ratio of time do you see yourself cycling in Oxford as opposed to cycling the varied terrains of the world?
I ask that because it's very easy to simply say 'buy a Raven Tour' as soon as one sees the word 'travel varied terrains', but this model might not be the bike for daily use.
My first thought would be that if you are investing for the long term, then seriously consider getting both bikes with the Rohloff 14 speed hub gear. This is the bit of kit that will last a life time if looked after, long after deraillieurs have croaked.
That narrows it down to the Raven, the Raven Sport Tour, and possibly the soon to be available Mercury. The Raven Tour is also available with a female spcific frame option, in I think two sizes. Broadly speaking, the Raven is a serious, specialist touring bike with greater strength, carrying capacity and weight. The RST is the 'fast touring bike' based on the Raven, but with a more road-oriented feel and a lower carrying capacity [tho' it can still be used for carrying], and the Mercury, well who knows, but it looks like an exciting 700c light tourer.
This is all assuming you're not going to be going everywhere together, in which case the tandem might be an idea!!
You'd have to check the brochures on the Thorn website
http://www.thorncycles.co.uk to work out sizing, but it sounds to me like your best course of action would be to ring them, book an appointment and go and see them and the bikes in Bridgwater; they will be very frank with you and will help you make the choices and order. You really cannot beat this approach in terms of getting the best fit.
Good luck.