Rodent breeding season is March to October, so it would be very early if it were a juvenile rat, especially when you consider it takes 3 weeks for them to mature enough to fend for themselves.
Having had both rats and mice (for the first time in the 50 years my family have owned this house) during the pandemic lockdowns, I think it’s probably a largish mouse. A body length of 10cm is typical of a mature adult mouse of most species (there are 6 in the UK) and in your photo, if the back legs weren’t stretched out it wouldn’t be a lot over that.
If you’re not sure on appearance alone, another way to check is their droppings. Rats tend not to just go everywhere (we didn’t find one dropping in the loft, despite hearing them careering around like elephants in clogs up there every night), but stick to set latrines to mark their territory, whereas mice, just poo prolifically everywhere they go. There are also clear differences in size and shape which are easy to google.
To be honest, as a pp said, where there is one there are more and if you book an appointment with a pest controller (some do a free initial appointment) they will know straight away what you’re dealing with. If you do, make sure they are properly accredited, especially if you are considering engaging them to use poison on your property. BPCA. If you decide to use human traps, you need to check them every 3-4 hours as mice can starve to death really quickly if left in one for longer, which kind of defeats the object of a humane trap.
Whatever you do, you need to do it swiftly as we are now coming into breeding season so what is a small-ish problem at the moment could get much worse over the coming months if left.