The 90 days start the day the Visa is issued and when you enter Thailand anytime during these 90 days you'll get a 90 day entry stamp. RTE London are one of the quickest for Visa approvals so applying one month before your departure is fine.
The financial requirements are the same as a 12 month extension so get that with a multiple re--entry permit for 1900B plus 3800B, the new Non-O from Laos costs 5000B plus you have the travel, Laos Visa and accommodation costs
A border run agent would not take you to the border if they could not get you back into Thailand with a new entry stamp. Many are denied entry at borders who have a history of staying in Thailand for periods without a proper Visa.