Someone may find this of use:
We had a brief visit to London yesterday to renew the wifes and stepsons Thai Passport.
The Embassy is having some building alterations so the Passport and Visa applications are handled just inside the main entrance.
I was amazed how easy it was given my recent visit to Liverpool for my Passport renewal.
They issue Visas at 11am so opt for an earlier slot on the online booking section of the E passport application online as it got very busy just before.
We were booked in for 10.30 and 11.00 but we just located the Embassy and my wife wanted the bathroom (No Toilets inside take note) so we went inside and no one else was waiting at 9.45am. They dealt with them straight away. It took about 50 mins to sort out the application for 2 including the PK 14 which i thought would be an issue as my boy has no contact with his Thai father.
Mad dash for a coffee shop toilet after :-)
Note it states £25 cash for the passports but take extra for Photos and return postage if not collecting.
Passports returned 4 to 6 weeks they said.