I shipped a TON of stuff to the UK over the past few years and never really had a problem. I always marked it as valued. For very small orders and team orders I marked it as a gift but with a value. Also on the customs form I just put RC Car parts and then a general description (ie Battiers, motors, brushes, etc). For yours I would just but Parts. But their customs are bad sometimes for wait time. The only one worse is Canada!!
EA