'Authentic' is a never going to be possible. The word relating to Thai food, changes with every region you visit.
Even with Chinese restaurants. If you visit Chinatown in London, there are restaurants that serve authentic Chinese food inc. chicken's feet, duck's tongue, pig's innards etc. which are frequented by 'ethnic' Chinese and SE Asians, who love it, but for most 'westerners' this stuff would be unpalitable.
Gordon Ramsey's recent TV series gives good examples of this.
I think the best use of the word 'authentic' in the UK is the knowledge that Thais are doing the cooking. The food has to be westernized, as you say, and any experienced chefs would know this.
Good luck in your quest.