UK Visit Visa is US$138, yes, you pay in dollars. Average processing time is within 10 working days after your girlfiend goes to Bangkok to apply and get her photo and fingerprints taken.
Before this get's locked, you have left it far too late if the wedding is this year ! To prepare a decent application, get an appointment for the biometrics will be very difficult, let alone getting a flight at a decent price.
If you don't plan on leaving Thailand for side trips, then a Tourist Visa from your home country is the cheapest and best option. If you plan to stay over 30 days the airline check in staff MAY give you grief if you do not have a visa.
Arriving in Thailand should not be a problem, however, depending on where you fly from and the airline used, they may not let you board without proof of a ticket out of Thailand within 30 days.
If you are coming from the UK, leaving there you need to declare cash if more than equivalent to 10,000 Euro. Even for amounts less than that, you may be questioned as to the origin of the cash.