OK, now we know that your problem is not so much with BaseCamp (you can plot a route, evidently) but with your using of your GPS device, I can suggest a simple trick for you to try. About a mile or so from your start point, in the direction / on road you want to take, insert a via point (ie a point you must go through) into your route. Depending on which GPS device you are using, when you fire up your route you may well see a choice of three destinations:
1. Start
2. The via point, a mile or so away from your start point
3. End
Chose number 2.
From wherever you a standing (whether that be Aberdeen or Norwich or Canterbury or Bath, it makes no difference, nor if your via point is in say, Penzance or Birmingham) your device will route you to that point, without fail. From there, the route will run normally, right to the end, without you having to do one more thing. There are other methods but try this simple one first, please.
What device do you use, by the way?
PS The only caveat I would make is that if the via point 2 is behind where you are standing, the device will route you backwards (so that you pass through point 2) and then the device will run you forwards along the route, in the direction you want to take.