Being based at a base not a million miles north of Northholt and having flow my Ac into said base, firstly you will need to ensure you have Crown Indemnity Insurance. You will then need to contact Northholt's duty Ops Officer and make your intentions known and also file a flight plan. If you are not MOD personnel then there may be some other hoops to jump through however, a quick call to Northholt Ops should see you right. Hope this helps.
