Lottie - you raise a number of interesting points - but I'm not sure how easy it is to draw conclusions from them.
With regards to Sacred Heart, the situation isn't going to be straightforward. By year 6, it is possible that a number of the children that transfer to Teddington School are non-Catholics that have moved in later years to the school simply for the Teddington School link. We don't know this information from the figures provided. What we can tell is that its location (and presumably that of most of its intake) has always been one of the worst for both getting in to (quality) Catholic secondaries and for transport links to them. If a parent of a child is faced with a choice of either St Paul's Sunbury or Teddington School - it's going to be a no-brainer which they'll choose!
It does seem more likely to me that the vast majority will choose a Catholic Secondary in Twickenham over Teddington school. (This does rather assume that its going to be a good school of course!) And if the Sacred Heart pupils are spread over a large geographic area then it can be seen as positive if most pupils can transfer to the same secondary.
The figures published on the school's website don't actually show a huge number transferring to the private sector, only 3 out of 25 last year. Although that might not be typical and be purely indicative of the current economic climate.
The question for me is whether the understandable wishes of a minority should supersede the potential uncertainty faced by the majority. The council is rushing this decision. There is no immediate need for a either a new Catholic or a new community secondary. Places in both sectors are currently in surplus, so it would make sense to simply wait and see. Buy the Clifden site by all means, but sit on it until it is clear what the priority needs to be.
I do get annoyed by the argument that "Catholics are taxpayers too" which I noticed also crept into the consultation document. It's quite a nasty little statement. It seems to imply that only wealthy minorities are important. It suggests that non taxpayers, despite often being the most vulnerable in society, can be simply ignored.