Bangkok

Some of the UK banks have low or no transaction fee charge deals on foreign currency.
I know the Post Office and Santander had them (not sure if they're still offered), maybe Halifax BS as well (as Karazyal mentioned). Have a search on line.

You could get a prepaid, but I never have, but I do carry 3 cards (one bank, one main CC and an emergency CC) all with different pin numbers. I don't use contactless and keep cards in 2 small separate metal card cases.

I also take a decent amount of cash purely because I started travelling before cards were accepted everywhere and I like to get the best ROE rate whenever possible (which is better for cash).
Also I use money belt and another personal 'hidden' carrier (plus the cash is in plastic bags as sorting through damp note because I've sweated through the belt, isn't nice).

Thanks so much Andrew! One random question I have, ruck sack or suitcase? Im in two minds!

Rucksack, definitely. And keep it small, like 40 litres. Don't make yourself miserable carting loads of gear around.

Everyone has asked how long you're travelling for, and you haven't answered. It’s kind of critical when you're getting advice on visas.

Ah I see! So many people have different opionions! I haven't answered the question because if im honest I don't know how long im going for! At the minute im looking at doing an escorted tour with a company called Contiki! x

If you show up at your departure airport with a one way ticket and without a tourist visa or a flight out of Thailand before 30 days is up you MAY NOT be able to leave on your trip! Never leave the home airport for your trip and you lose your hotel reservations and any fees you pay for your tours.

With a definite date for a return flight home you will be able to plan a daily budget out easier. You can't work in Thailand, at least nothing legal, so what money you have at the beginning of your holiday will have to last the entire trip!

Up to you!

Quoting GeorginaSeddon

im looking at doing an escorted tour

Why?
Thailand is THE easiest country in the world for backpacking.
Loads of 'independent' travellers, Hostels, Guest Houses, cheap Hotels, Easy travel by bus, train and Air.
Every backpacker place (hostel, bar, eatery) and pretty much every non-backpacker place has loads of travellers of all ages, all happy to tell you where they've been and where they're going, and how they did it/will do it...
But you're thinking of paying someone else to arrange something you can do easily...

As Karazyal says, look into the Visa situation as you only get 30 days on entry, and their immigration/visa situation has changed very recently so you will have problems if you overstay.
For many years you just had to pay a fine but now you're likely to be blacklisted (can't re-enter TH) for some years, and once you've been there once you will most likely want to go back again regularly.

[ Edit: Edited on 12-Jan-2019, at 10:13 by Andrew Mack ]

Quoting GeorginaSeddon

Ah I see! So many people have different opionions!

Well they're all telling you the same thing - Thailand isn't somewhere you can visit on an open-ended basis. As a walk-in visitor you get 30 days and need Proof Of Onward Travel or you risk not being admitted - and in some cases your airline may want to see it before allowing you to board, to protect themselves from fines and/or being liable for shipping you back home.

Do you plan to move on to other countries after Thailand? Is this part of a longer trip? Or is it Thailand and home?

If you're moving on elsewhere, you could get yourself a cheap flight to your next country. That would sort your POOT. SouthEast Asia has budget airlines like Europe does, so you may find a short flight with AirAsia or Tiger for under £50. Even if you end up not using it, and going elsewhere, it saves you a lot of trouble.

I'd second what Andrew said about Thailand being easy to do on your own; there are millions of backpackers there so everything's set up to cater for them. It's a very cheap place, so while a Kontiki tour would be a way to meet people, just staying in hostels would achieve the same thing at a fraction of the price.

You can find out info on longer a visa on the Thailand Embassy UK web-site, but this is the brief info on a 30 day 'on arrival' tourist visa (Which says you need Proof Of Onward Travel); http://www.thaiembassy.org/london/en/services/7742/84451-Tourist-Visa.html

It also mentions having adequate finances, but the amount is tiny (Baht 20,000 = roughly £500)

[ Edit: Edited on 12-Jan-2019, at 11:00 by Andrew Mack ]

"One random question I have, ruck sack or suitcase? Im in two minds!"

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Not sure what your experience is with flying. There are limits on what you can take into the cabin area of your airline. If you intended to bring big bottles of lotion, perfume, sun tan creams or other liquids into the cabin area they will be confiscated at the security check. Bigger bottles of creams, lotions, perfumes can be in a checked in bag but not carried into the seating area. Size matters!

If you do check a pack in I know some travelers stick the pack in a bag to keep it clean and not be caught on machinery. Or at least tie the straps down or pack them inside so they don't get ripped off on carousels.

Your airline will have guidelines for what you can bring into the plane. There are other security checks in the "in between" countries. Also if taking certain medicines with you there are restrictions.

https://www.gov.uk/hand-luggage-restrictions

Up to you.