Sevenoaks is a town in England.
Hitching onto the M25 is reasonably easy, but don't make the mistake of going to junction 5 as it can take hours to get a lift. Head to Clacket lane services, just past Westerham. These are either accessible via a bus to Westerham and walking along the main road until you reach Clacket lane or you can hitch along the A25 reasonably easily. Once at clacket lane walk along until you find the services about half a mile down the road. Hitching either way is usually quite quick, although if heading north it's advisable to initially hitch east and get to thurrock services and sign m1 from there.
Either hitch to clacket lane and then thumb along the A25 or find someone going down the A21 as it's the only way to get off without being on a motorway junction. Getting off at the base of a bridge if someone is willing to stop nearby is usually quite safe though, police in Kent are generally quite relaxed.