Hi Becky welcome to the forum from Shetland.
I have bought some stuff in the past from Ultralight out door gear. There are cheaper places, occasionally, if you shop around but they do free postage, even to the Northern isles and they often have a deal where you can get another item cheaper along with your purchase eg water proof stuff sack.
they have an extensive range of sleeping mats and do their own reviews off them. I have phoned them in the past and they were most helpful. If you check the link attached you will see some very cheap air mats like the multi mat from £29.99 at 440 grams.
or one of the big names like Exped AirMat HL M Regular Sleeping Mat coming in at £99.99 304Grams £15 below RRP free postage. with a half price black diamond head torch deal. R Value 1.5 which is maybe as cold as you would want to risk. Although if your tent floor is super water proof, you can lay some clothes under your mat and up its R Value.
https://www.ultralightoutdoorgear.co.uk/equipment-c3/sleeping-mats-c58/ultralight-airbeds-c138