We decided to do a bit of research though and contacted a few people as well as simply googling to see what we could find in terms of heat killing gluten, we wanted to actually run an experiment but have yet to find anywhere that can help us conduct the experiment since it seems to have never been done before (feel free to contact us if you have access to a lab and can help us with this!)
First off gluten is a protein molecule, you can't kill a molecule. Simple biology. So when someone says heat kills gluten, they are way off the mark, now if they say heat destroys the gluten that might be slightly different but still not entirely true.
When we spoke to Tricia Thompson, a nutrition consultant, researcher and writer from Boston who specialises in gluten free and is the creator of the website Gluten Free Dietician and founder of the company Gluten Free Watchdog she provided us with some interesting information which was in an article she wrote. Now they haven't conducted an official test at their lab but Thom from the lab that runs all the tests for Gluten Free Watchdog does provide some insight:-
As far as gluten being broken down by the heated oil, this is unlikely. According to my Joy of Cooking cookbook, the best temperature for deep frying is 365 degrees F. According to Thom, this is not a high enough temperature to completely hydrolyze gluten, at least in the short term.
"Gluten proteins are extremely resilient and can't be broken down easily with temperature or time. The oil might have some effect on the protein tertiary (i.e., 3-dimensional) structure, BUT remember some breads are cooked at 500 degrees F for 10-15 minutes (pizza) and the gluten remains intact. So the short answer is that hot oil for the most part can not be trusted to completely hydrolyze gluten."
Except from Tricia Thompsons "French Fries" post
To hydrolyse gluten means to break down the structure, while the heat may damage the gluten, it isn't hot enough to destroy the gluten molecule completely.
When gluten is subject to high heat a process called "denaturing" happens to the enzymes, which basically irreversibly changes the shape of the enzyme. Unlike other proteins though gluten is very tough and hard to denature, gluten can be broken down to it's primary structure (first breaking down the quarternary, tertiary and secondary structures) but it still remains a gluten molecule which is still gluten at the end of the day.
According to at least three articles, the one provided to us by Tricia above, this one which is actually about grilling, but same concept about heat applies and this article with information provided by a chemist, all claim 500F/260C or even higher (650 in the last one) is needed to destroy gluten. They may all be American but same information applies in the UK too.
This is also the reason some sourdough bread's are believed to be gluten free after cooking even though they are made with wheat flour, because they are cooked at a high heat for 500F/260C for a 15 minutes. While it's possible the gluten could have been destroyed in the heat would you risk it? I know I wouldn't.
In terms of the fryers destroying the gluten, if it takes over 500F or 260C to denature the gluten molecule fully the chip shop fryer must be hotter than normal as the heat of the fryers we asked about and found online were only between 130C and 200C (392F highest which is over 100 degrees lower than that needed for full denaturing of gluten). While this heat may destroy some of the structures of the molecule its not enough to make it safe for coeliacs, the gluten would still be gluten.
While there is not definitive studies on the degradation of gluten in fryers I think based off the information from dietitians, chemists and lab technicians provided in the articles it is safe to say that heat doesn't destroy gluten, or at least destroy it fully, so if your chip shop says "oh the fryer is so hot the gluten is destroyed", make sure you turn round, walk out and find a chip shop that uses a dedicated fryer!