A plan for getting to LHR,checking into a hotel there and, then, making the short trip to Windsor is really the best option. But, it's getting to LHR or for that matter almost anywhere else from BOTW that is the problem. An idea although an expensive one would be to hire a car service from BOTW to LHR. From Oxford this costs about £100. Have no idea as to cost from BOTW. There are a number of taxi companies in the general area which offer this service.