The thatch isn't an issue at all. It's very warm and naturally sustainable, and costs around £10k per side every 25 years or so and round another £5k for the ridgeline. I'm not sure that's far off tiled roof costs over time.
What happens if it was last done 24 years ago, though? People balk at buying used cars with overdue new cambelts, but that's a tiny fraction of the cost of re-roofing a house.
Also, with tiles, you might have to replace the odd one now and again, but I don't know anybody who budgets/pays £25K for a new tiled roof every 25 years. Also, if you do need work doing to your tiled roof, there's no shortage of general roofers out there who can do it for a fair market price.