BBQ gas bottle

Where I am in the South West it was pretty difficult to get hold of a new bottle through official suppliers (Calor). I had to buy two empty bottles from Gumtree/Marketplace to then be able to take them to get them swapped with full bottles. I think I paid £5 each for 13kg patio and 15kg Butane.

Definitely give that a go. If you do find anywhere with stock and allowing you to purchase one the cost will be much more for the deposit on the bottle which you'll probably never get back.

Somewhere local?

I've always found Flo-gas from builders merchants and hire shops to be slighlty cheaper than anything sold at garages etc.

Marketplace

Try market place

Ebay, also don't be too bothered about buying the fancy expensive patio gas bottles as any gas bottle can be used on a BBQ this time of year you simply need to change the regulator which costs about £5, usually the Calor blue Butane bottles as used in room heaters are easily obtainable and also cheap.
But if you intend to BBQ in the colder months / Winter you will need patio gas or a propane cylinder because of the lower temperatures freezing the gas. (edited)

Try Here

I’ve used many times, always been good

Always worth trying the tip (aka recycling centre). They're not meant to, but depending on who you speak to, they might let you take one away.

And if they do, make sure you pick a heavy one (edited)

As far as I know no outlet with starts a new contract on any cylinder type, only exchange like for like.

Buy a used bottle off marketplace/eBay/gumtree and exchange that for full cylinder.