Congratulations on the excellent running race! Also, it’s OK not to step up to 10 km if you enjoy doing 5 kms. You can do them for a while, exploring new areas in that distance (because lots of places have races that length). I’d also add that Oulton Park has an off-season running race (this year’s is December 2nd) that has several choices of length, and I think the shortest on offer is indeed 5 km.

If you do decide to go 10 km length (to get into the Silverstone race or any other reason), stepping up gradually is perfectly fine. You will get faster with practise, and adding time on in 5-minute blocks can be a good way to build up without shocking the system (or your sense of time allocation) too much.