Quick Version
I don't know about the flavour.
As long as you are using it with a base malt you can use up to ~50%, you don't need to get malted triticale or pre-cook it(see below). Also, I have no idea where you could find some in the UK; if you can find some flaked then try that.
PS: if you know where to find some please post a link here, because I am now rather interested in trying it too.
Extended Info
I currently know nothing about the falvours, but regarding the techniques I have found a reference. Brewing: New Techniques edited by C Bamforth Section 3.4.1 Unmalted Triticale:
"Most non-malt adjuncts do not contribute either enzyme activity or soluble nitrogen ... triticale goes beyond this specification ... Because of this and the low gelitinisation range of triticale starch(59-65C)... triticale could be used as a brewing adjunct at high ratios(<30-50%) ... furthermore, because of its low gelatinisation temperature, triticale can be added directly to the mash tun, without the need for a cereal cooker or second mashing vessel"
https://books.google.co.uk/books?id=gcJQAwAAQBAJ&pg=PA43&lpg=PA43&dq=gelatinisation+tricalate&source=bl&ots=u-8qrBrmGl&sig=e0n5SjJQ22lwfnp6Y97pBMjFMXU&hl=en&sa=X&ei=e0yWVYy2H6zg7QaosZyQAg&ved=0CCwQ6AEwAg#v=onepage&q=gelatinisation%20tricalate&f=false