This is what the law has to say -
http://www.hmso.gov.uk/si/si1989/Uksi_19891796_en_1.htm
I interpret this to mean that neons are not legal if used on a road as defined under the road traffic act.
A road is defined under s.192(1) of the Road Traffic Act 1988 as “any highway and any other road to which the public has access, and includes bridges over which a road passes”
A highway is a way over which the public has a right to pass and re-pass by foot, horse or vehicle, or with animals.
At worse i would image it is a convictable offence under "CU60 Undefined failure to comply with Construction and Use Regulations" which will result in a fine, penalty points on your licence and increased insurance premiums as a result.
There is no specific law on neons, but if the use of them continues to rise i wouldn't think it will be too long before one is introduced. The act isn't amended very often (as you can see, the last one was 1989).
It still remains a grey area, basically we need someone to get caught and be prepared to spend and absolute fortune trying to defend it. Then there will be a case law and definative boundaries written down. Unless you are prepared to do this or are happy to accept a potential fine, a rectification notice (insisting they are removed) and even points on your licence under the construction and use offences (which will in turn increase your insurance premium), i would strongly recommend not having neons (whether tubes, washer jets, tyreflies, no. plate screws etc).
I hope this clears things up a bit for you all.
*Credit goes to Shiny @ the Max Power forum