I'd say the A14 is possibly going to be a good option, there are several bridges over that, that are good for light trails, most of the M1 round there is illuminated which never really allows for brilliant trails though I did once stop at Watford gap services for some off the footbridge there, don't discount street level trails around parts of the town centre
You might find it useful to give my guide to light trails a good read it should help you with shooting light trails...