You seem determined to invite prosecution ;-)
While the Highway Code isn't, in itself, a legal document, it provides guidance regarding reasonable driving standards. The minimum stopping distance, at 50mph on a dry road, is given as 13 car lengths. However, stopping distances should be doubled on a wet road, which takes us up to 26 car lengths. Additional distance needs to be added for vehicles with heavy loads. That suggests that the minimum stopping distance for your vehicle was probably greater than 30 car lengths.
The Highway Code also states that drivers should always drive at a distance from the preceding vehicle equal to, or greater than, the minimum stopping distance for that speed. So, unless yo can convince a court that you were at least 30 car lengths behind the preceding vehicle, you're inviting conviction for 'careless driving'. If you could convince the court that you were the correct distance back, you'd find it hard to explain why you changed lanes. Remembering that you should always be ready to stop at an amber signal, you seem to have little chance of winning any court case.
Chris