Ignore the peanut brains trying to give you grief over a legitimate question. They look for people to put down and don't even pay attention to the name of this group.🙄
As others have said, you will be fine without a visa as you're a UK passport holder. Now, as Ronald Tan mentions above, Thailand looks to be implementing an electronic travel authorization requirement which isn't as intimidating as it sounds, but it is an extra step you need to know about. We don't know the details yet, but you will have to go to a Thai government website and fill out an online form in advance. Will probably not be too time consuming or onerous. The US already has it for her visitiors and the UK will have one, too soon. Just something that comes with the times. But keep an eye out for this so you know by when to go online and submit it.
AND that agent is also probably trying to set himself up for recurring fees every year. Some people don't want to lock up the 800k and that's fine if they'd prefer the lower but recurring fees. But in the event the agent is pushing that, doubly sleazy.