If you're going to use ANY bottled gas heater in that situation it's ESSENTIAL that you ensure that there's a good flow of air into the chimney AND that you've got a carbon monoxide detector in the room. (They're cheap:
https://www.screwfix.com/p/dicon-co-di-9br-carbon-monoxide-alarm/2117g )
Clarke products have frequently been praised here on AB. They're sold by Machine Mart, which has branches in almost every town in the UK. So this might be worth considering:
https://www.machinemart.co.uk/p/clarke-mobile-gas-heater-mgh1/
When you first purchase a gas bottle you have to pay a hefty deposit on the bottle (which remains the property of the gas company). Thereafter though you simply exchange the old bottle for a new one, with the deposit being retained against the new bottle. Calor's current prices for a 15kg butane bottle are £77.99 for an initial purchase and then £38.00 when you exchange an empty bottle for a full one.
The information here might prove to be useful:
https://www.calor.co.uk/help-and-advice/calor-gas-heaters/types-of-calor-gas-heaters