Ref the LAST device..
As i understand it the problem with the LAST device was that due to low power output (being Battery powered) the signal would only go out a short distance (approx 30 miles) where as the rules that define what a transponder is are really aimed at the Commercial airliner world which requires the signal to go out hundreds of miles. And at the time the CAA therefore would not certificate it, but things have now changed and certification is controlled by EASA...
There was mentions at the Friedrichschafen event earlier this year that a UK manufacturer was going to release a certified handheld device by the end of the year for around £500.00 and that its development was being sponsored by NATS...
To do this there was mentions that Eurocontrol were going to change the rules so a lower powered device for general aviation would be allowed.