So the plan is we fly a smallish jet at 400 mph from Southampton area as low as possible using Gatwick as its turning point.
Why bother - just hi-jack a 40' artic from Tesco and fill it with "stuff". Fake ID, take the Tesco driver's overalls, and you are in.
We did lots of these type of scenarios in the late 70s and early 80s in a certain green outfit. Rarely did we fail to get onto the target with enough "stuff" to knock out an essential component, and we'd only done the most cursory of recces. Google Earth, some good quality camera-work and plenty of ground recce and unless you have the Brigade of Guards standing shoulder to shoulder all round the target you'll still get in. Even then a few flexwing microlights would do the job, especially at night.