I if was to hazard a few guesses:
1. Its the first event and first events do not historically get many teams from far and wide. (see Norway, Catalunya, Norway, Madrid, Malaga...) Next year will be different.
2. Its the end of the year, and many teams have players with no money, or no holiday left, or both.
3. Most of the teams who travel to foreign events do not focus on 5 man, and would not waste their time going to a 5man event when holidays and funds are better spent on 7man MS events.
4. Some of the top teams who travel will be engaged in San Diego. (Shock, Nexus and Tigers)
5. It has not been particularly well publiscised. It is possible that few people in the UK were actually awareof it existing.
Any of those float your boat?