I love Goa too, but Thailand is absolutely fantastic, would definately do the 4 nights in Bangkok, there is so much to do there, then I would recommend the Krabi area which is great, especially Ao Nang.
From manchester most tour operators use Qatar airways, Emirates or Etihad which all change in the middle east.
Hope this helps , shirley

Train travel in Thailand may save you money, but there are also plenty of low cost air carriers that make flying around Thailand pretty cheap these days for example Airasia, Nok Air and others.

You can definately both eat for less than a tenner in Thailand. As previously posted a lot of the internal flights are very cheap - we noticed Bangkok to Koh Samui for £50 with Bangkok airways. I know that the ferrys are dirt cheap. £1.50 from Koh Samui to the mainland as one example. Bangkok is a wonderful place - it buzzes - and you cant explain what its like to anyone who hasnt been really - the smells, the noises etc are fantastic.

I agree internal flights are cheap.
But not as cheap as trains.
We looked at travelling by train to Koh Samui this summer.
Cost £20 for 1st class sleeper. This is a private 2 birth lockable compartment with wash basin and air con. You could look at travelling overnight as this will save a nights accomodation. (takes approx 12 hours).
This was the most expensive, there are cheaper alternatives.

In the end we decided to fly because of the time we land in Bangkok, and because we got a good price through our agents.

We got the train to Hua Hin last year and it was a great experience.