I imported a pair of gel blasters from Australia at the begining of the year, and I had no issues. The operating principle is no different to airsoft pews here. However, airsoft is banned in Australia, and "gel blasters" have become a popular alternative. In all states (except Queensland, where I was) a licence is needed to hold a gel blaster... Strange I know...
A P90 SmG and a Glock G17.
I of course was concerned about this exact issue, so I contacted Customs here both by phone and email from Australia to try and figure out what my steps were to be able to import them.
I was told that there is a comodity code that covers air weapons (9304 0000 00) which covers the importation of airsoft weapons with customs. I had no issues whatsoever and they arrived literally no more than a week after I got home from Australia. I was never even contacted by Customs or the police about my pews...
If you are carrying them in your own lugguage, you first have to tell the airline you are carrying them (don't use Singapore Airlines, or you will be prohibited from travelling, this is why I had to ship mine with a specialist freight company at a cost of £772 at the time as I paid in AUD), and if they allow you to carry them, place them in your checked lugguage and declare them to customs when you arrive in the UK. They may inspect them to make sure they are not real firearms, however that should be it.
I hope that this info helps in some way in future.