I see that you have another post elsewhere on the forum asking about refillable lpg bottles.
Gasglow sell re fillable gas systems and bottles
http://www.motorcaravanning.co.uk/shopuk/gaslow_refillables.htm
I'm not sure how much the initial set up cost is or how often you use your caravan but I'm not sure where you get £25+ for a 'normal' lpg bottle from?
Calor gas bottles are used on a kind of hire basis and when they are returned empty they undergo meticulous safety checks which would account for some of the price differential.
My non-refillable Calor butane 7kg bottle costs £15.25 from a hardware shop close to where I live, it's recently gone up from £12 last time that I bought one. The bottle can be exchanged pretty much anywhere in the UK although caravan sites and dealers accessory shops do charge more.
Some lpg butane such as Citygas can be bought for £11 for a 7kg bottle from certain Asian shops if you know where to look, I stick to Calor because of the convenience of being able to buy a new bottle wherever I happen to be.
We use far less lpg during the warmer months because we rarely have the caravan heater on and because we go on lots of caravan rallies with no ehu (electric hook up) we use more gas than most caravanners who don't rally and who can have their fridge and water heater running off the 240v rather than lpg.
Refillable lpg bottles should only be filled to 80% capacity and concerns have been expressed about the safety of the system as a whole.
http://www.motorcaravanning.co.uk/pdf/Anyone_for_a_refill_CC_Jan09.pdf
John Wickersham is a very well respected authority on caravan maintenance and safety.
I'd try normal lpg for the first season to see how you get on because you might not be much better off but you must weigh up the pros and cons for yourself.