Immigration officers are only really there to process visas, maybe in a quiet immigration office they may give you a short period of time with some suggestions, but it's ultimately your decision, as to what visa is best for you, do some research as to the most suitable visa, based on age and circumstances, either that or just use a recommended visa agent, that will do the whole process for you, but remember that they are a business, they may not necessarily advise the best visa, but a suitable visa, that they can also make money on getting it issued to you.
as long as you meet all the requirements your visa application will get approved, sometimes they ask for additional documents, you need to print the E-visa and carry it in your passport, it has an issue date and a use by date, you can enter Thailand anytime within that date to activate the visa, and you get stamped in for 90 days, the London embassy are very quick at issuing e-visas, people report getting them usually within a week, so a month before would be okay, I've send an information link in another comment, that explains the way to apply on the E-visa site.
Apply for the NON O visa on the Thai e-visa website in the UK, before arriving in Thailand, try and open a bank account yourself, it may be possible with the non O visa, get a visa agent to open the bank account if you can't do it yourself, and deposit āļŋ800,000 that needs to be seasoned for two months, when you arrive in Thailand go and get a list of requirements for a "12 month extension of stay based on retirement" after 60 days in Thailand you should be able to apply for the extension, the new DTV is another option to look at.