The fact that EADS are sponsoring RIAT could well have a huge influence on the fact that the 109 is appearing there.
I have had no hand in organising a display so can’t speak from experience but there must be a huge number of reasons why some of the Europen warbirds don’t appear here including fuel costs (balanced against the fees for a display), possibly insurance restrictions and it could even be that some are not keen to risk a channel crossing (even though the channel is a comparatively narrow stretch of water) where the consequences of a technical problem could be an almost certain total loss into the sea as opposed to there being at least some chance of a successful forced landing or diversion to the nearest airfield if the aircraft had problems over the European mainland.
The above is only my own personal view based on a common sense approach but it does seem a fairly logical explanation for the situation.
Incidentally although I wasn’t lucky enough to see it I think TFC deserve huge credit for bringing the Caudron Racer replica over last year for what I believe was its only UK display. It was a great pity that it didn’t also make it to Old Warden while it was here as that was probably the most appropriate UK setting for it.