How long is a piece of string? It depends on so many factors, ie time of day, number of passengers, number of flights, number of staff, flight delays etc, the list is almost endless!
You wont obtain a 1 year multi-entry marriage visa in Thailand. You will obtain a 90 day Non Imm O visa and then apply for a 1 year extention of stay and a multi re-entry permit. This will give you 1 year and 90 days in Country and the total cost should be 7700bt
You can do as many border crossings as you like it's a multi-entry visa. The restriction to 2 land border crossings only applies to people without a visa.
Get the Non Imm O Visa from RTE London. That gives you 90 days to open a Bank account and season your 800k for 2 months prior to applying for your 1 year extension of stay. I dont think you can open a BB account from the UK, however there is a London branch so maybe check with them.