UK has visa exemption for 60 days. You will have to fill out the new online arrival card 3 days prior to arrival though (at least 3 days before arrival, but access isn't available until the 28th of April i think)
I'm pretty sure it's 3 days before your intended arrival. It might be causing problems because it hasn't been officially implemented yet. I guess after
The ask:thailand community, consisting of multiple Q/A groups with over 100,000 members, powers this platform. It is not an official government resource. Our members actively contribute to this resource, and while we strive for accuracy, we cannot guarantee its complete reliability. Assistance to travelers is provided as a community service.