Jonathan,

There is a minefield awaiting you if you really want to look at this in detail.
The principle behind these products is that they create a 'low emissivity' layer by reducing radiation losses by virtue of the 'shineyness'.

However there are some caveats; the Low-E effects are measured in still air, and the ventilated cavity on the outside of the frame is not still air!
The other issue is the testing. The manufacturers have these products tested by TRADA.
U-values should be calculated according to BS EN ISO 6946, and the TRADA tests aren't, afaik?

To summarize, there is a benefit in using the reflective breather membrane, but how big that benefit is questionable.

Good luck...

A number of points to address.

1) If you are going for excellent U-Values the calculated improvement gained by the breather membrane is small (can be tiny at very very good U-Values)
2) It only works with masonry cladding
3) The cavity between the timber frame and masonry cladding should be slightly vented (in accordance with BS 5250), this will reduce the performance of the membrane over an un-vented cavity as calculated (in accordance with BR 443)
4) Glidevale TF200 thermo is not tested by TRADA. It has been tested in a hot-box by a UKAS accredited lab.

Now, they do offer benefits other than thermal, TF200 thermo is guite a robust membrane with good tear resistance, so easier to handle, and less prone to damage.

It really depends on the price premium and what insulation values you are going for. As with all things insulation wise, the law of deminishing returns kicks in. If you insulate to a U-Value of 0.15 or better, the addition of a reflective breather membrane is unlikely to improve that (unless you are on the cusp of rounding). If you are offered it free, take it. If you have to pay any decent amount of money, don't! Put more loft insulation in instead.

I have an I-beam timber frame with 15mm OSB3 outer racking. It is covered with Glidevale TF200 Thermo membrane which was a standard fitting. It's been up for 18 months with no protection and has not deteriorated. As a tough membrane it is good whether there are thermal benefits I would be doubtful.