Campingaz is very expensive. From what we read here, in the UK even more so than anywhere else.
Just know: there are two types of Campingaz bottles around, the 'regular' which is only suited for butane, and 'dual use' ones which are also suited to hold propane (albeit only 2.5 kg instead of the 3.0 kg of butane, for obvious reasons). You can see on the bottom rim of the bottle if it may be used for propane (see photo below).
Where professional gas refill stations usually won't touch the Campingaz 'butane only' bottles (they just swap them), in our experience the 'dual use' bottles can be refilled with propane at a professional gas station without ado (you may have to point out the notification on the rim). We have done that in Italy and Norway this year.
Just had ours topped up (another nice possibility, compared to 'regular' Campingaz...) again this morning at a local station in our neighbourhood here in NL, for our upcoming Sweden trip. Cost: €6.63. Really a lot cheaper than any Campingaz swap, anywhere.
We have seen the 'dual use' 907 bottles pop up in Germany and the Alps region. Just look out for them whenever you do a swap there. We scored ours in Italy, last year. Or buy a new 907-sized propane bottle for yourself, ie.
here or
here. After one or two refills instead of swaps you will have recouped your initial investment already!
Then, of course, it may not be a wise investment for UK-based members: there is always the question whether, after Brexit, you guys are allowed the continued use of these horrid EU-standard bottles in the UK at all... Guess Boris and his crew will want you to go back to Whitworth threading, psi norms and other things that made Britain great once! (pun intended!)