I know Bexleyheath very well, small world.
Unfortunatly, i'm not too familular with Sevenoaks in terms of bike shops.
However, I know a few places, depending on what you're looking for. There's J&S accessories in Eltham for all your clothing, helmet and like you say generic Oxford stuff!https://goo.gl/maps/nprXxcJstnBoL5ZT7
For more specific parts, I go to the main dealer unfortunately.
There's a Kawasaki shop in Rochester which seems reasonablehttps://goo.gl/maps/46APgNW8i6EjYnKf7
but I go to Laguna motorcycles in Maidstone.https://goo.gl/maps/YfDTanz4q6gStCot9
There's a Sondel in Blackfen, there a Yamaha dealer, but I know someone who get's their Z1000SX serviced there.https://g.page/SondelSport?share
Hahaha, just realized it's called Sondel sport on Google and says it's a water-sports retailer, I've never seen anything other than motorcycles in there and it's a Yamaha top dealer! Might call them and ask for a snorkel
John Riley motorcycle's in Crayford is a good local independent shop that does parts, sales and servicing. Doesn't look much from the outside but the mechanic knows his stuff and the owner has been there with the shop since the 17th century, probably the last time it was renovated too!
I bought a bike from them back in 2013 and had no issues with it, even replaced the rectifier under warranty for me too! Servicing and parts we're cheap. My next door neighbor goes there regularly still, he commutes daily and I've never seen either of his bikes broken down. I'd go there still, if I didn't have a bike on PCP.https://goo.gl/maps/9Lj4mZuggaNJq9ka7
Tyres and chains, I use Watling tyres in Swanley. There's a few branches of Watling tyres about, but not all of them do motorcycles. The ones that do are very well priced I find and good service. i'm due to get some Michelin PR4's fitted soon.https://goo.gl/maps/9tdAH9T8kPYNRxtq8
I've never had one here, but they do MOT's too...
But if you want a errrr...how can I say this...
motorcycle MOT specialist
then look no further than May's in Dartford.
Again the shop has been there since the year 0 and the old boy is very ''knowledgeable'' and will give you a very honest old school MOT test https://goo.gl/maps/GJ7WgT5cJqFemdMQ6
But I haven't had to get a bike MOT'd for many years now (I swap every two or three years)
Anything else I get online, usually from Sports Bike Shop https://www.sportsbikeshop.co.uk/
Hope some of that helps.
On another note, If you're thinking of joining the National Versys Meet this year, I'll be riding up and posting route details soon. If you want to join, you're welcome.