-10C is pretty severe for the UK unless you are bivvying out in the Highlands. A bag rated to that sort of temp won’t be much use for probably 11 months of the year. I’ve a Rab 1100 which is rated to -25C and which I’ve used for perhaps 20 nights in 20 years, nearly all of them outside the UK.
A lesser rated bag with a liner or second bag to get you to the required rating might be a better choice. I’ve a PHD sleeping bag rated to 5C and a Cumulus quilt 150 with the same rating, together they are fine down to at least -5C. Weight and cost are roughly equivalent to the same rated bag.
If you want top quality then PHD (preferably in the sale for your wallet’s sake) but Alpkit is a decent budget option. I would be tempted by the Minim 500 linked to above though despite my comments above.