Garmin states on their website:
Similar to GTX 330, GTX 328 includes features such as displaying outside air temperature, altitude monitoring, count up and count down timers (using a built-in digitized voice annunciator to alert the pilot when preset altitude limits have been exceeded or timer expiration), density altitude functions and front-panel input for flight ID.
Since the GTX-330 Mode S Transponder provides TIS (Traffic Information Services), EHS (Enhanced Mode S) functionality and transmits power that exceeds the requirement for VFR aircraft in Europe, the European GTX 328 does not offer TIS, EHS or ADS-B functionality.
GTX 328: Meets European flight regulations without breaking the bank.
This is what has been mentioned here before. The only question therefore is: Does UK CAA have another 'special rule' here.
So, to be 100% sure, i would seek written confirmation from the UK CAA. Besides sending an email to mode.s (at) caa.co.uk you might want to check:
Mode S Home Page | Mode S | Airspace Policy and
EUROCONTROL - United Kingdom
Edit:
I am pretty sure you will find an B206 owner here, who did recently upgrade his avionics and used a GTX-328 in it (i'll send you the reg. by PN)