i watched the opening night video...the casting seemed off and it looked like they were trying to "modernize" the show with the costumes. probably not a good idea to mess with a formula that has worked for 50 years.
It's ALWAYS the wrong time to open a musical in London. Unless you're based on a film and/or star TV/music personalities - or you're a revival of an established classic, which FANTASTICKS is definitely not in the UK.
Shows opening in the Summer rarely seem to do well in the UK, don't know whether its because most Londoners are on holiday away from London, or whether its the large number of tourists flocking to well known shows. Its a bit of a non-descript time of year really, makes it a bit difficult to publicise and market your show. This year is especially bad as its the World Cup, they're even using some theatres to show the matches (although luckily not any West End theatres)