I would not have thought it sensible to take a large amount of cash. Better to open a bank account in Thailand. I understand that HSBC have presence there.
The new rules apply if you are travelling to or from a country which is outside the European Union (EU). You must declare any cash you are carrying if it is 10,000 Euros or more (or the equivalent in other currencies).You could face paying a �5000 penalty if you fail to declare the cash or if you give wrong or incomplete information
As JK2 states, you must declare large sums but there is no limit on the actual amount that you can take out of the country. If you win this Friday's Euro-Millions lottery outright (with an 88 million pound payout), you've got every right to take the whole lot, in cash, out of the country.
You can take any amount you like out of England provided you comply with any necessary formalities.
Thailand MAY have import restrictions - you'd have to enquire about them
A bank transfer is the safest, simplest, and probably cheapest way. Even if you pay a fee you'll get a better exchange rate than cash, which for property purchasing quantities is highly unwise anyway.
If you want to buy a property legally in Thailand, you need to have evidence that the money came in from offshore. Only a bank can provide that evidence (they are used to completing the official paperwork).
If you are buying a condo or a company to hold a landed property, then having evidence that the money came in helps if you ever want to get it back out again. Otherwise, certain restrictions may apply on how you can get it out again