Because if you have a visa, then showing that visa is a required part of your application. Most foreigners don't know the correct terms for various parts of the immigration process. If I was an employee at immigration I would not answer questions either without seeing the person's passport so I know exactly what type of stay they are on and what the correct information to give them is.
Visa on Arrival, Visa Waiver, Visa Exempt, Tourist Visa, these are all different things but people use the names incorrectly all the time.
Some say I must do the 𝟗𝟎 𝐝𝐚𝐲 𝐧𝐨𝐧 𝐢𝐦𝐦𝐢𝐠𝐫𝐚𝐭𝐢𝐨𝐧 𝐎 (evisa) before I come, then change to 1 year retirement visa after I'm there.
Others say I can come on the normal visa (60 day tourist visa) then once there I can convert to the 𝐧𝐨𝐧 𝐢𝐦𝐦𝐢𝐠𝐫𝐚𝐭𝐢𝐨𝐧 𝐎 𝐯𝐢𝐬𝐚 then convert to the 1 year retirement visa.
First of all, there is no such thing as being on the same visa or adding someone else to your visa. Each person has to have their own visa. The way the DTV works is that one person can be the primary visa holder, and they must apply for and pay for their DTV. Then once they get it, a dependent DTV can be applied for based on this initial DTV, and one of the requirements is to upload the primary DTV as part of the documents. Each one must be applied for and paid for separately, and the dependent DTV is only available to spouse (legally married) and children.
Whether your daughter requires an education visa or not is up to the school. Some will want to know the child is either not on overstay, or for them to be on an ED visa, and some won't care either way. Most people with children that age do not do anything with visas and just let them go on overstay as children under 14 cannot be fined for overstay and children under 18 cannot be banned for overstay.
I assume you are asking about the one year multiple-entry non-o visa based on marriage. This is not available from any embassy that has switched to the e-visa system, so you cannot get it in the UK. In fact there are only 2 known embassies in the world that still issue this visa and it's the Thai embassy/consulate in Savannakhet, Laos and the one in Yangon, Myanmar. But they both require proof of 400,000 in a Thai bank account now, as they no longer offer it without proof of funds since months ago.
Your only other option if you don't want to show money is the multiple-entry tourist visa. It's a bit expensive and it only lasts for 6 months, and will give you a 60-day entry stamp each time you enter Thailand. But there's not much use for this visa since the visa exempt was increased to 60 days.