Transport
Blogs all over the warn of the horrors of crossing the border at Poipet, but MoveToCambodia provides an excellent post on this. They recommend a very cheap option ($11.25) involving a 2 hour mini-bus to Poipet, and a 5-6 hour bus to Bangkok. They do warn however, that this is a trip that will take you all day, and you are advised to leave early. The link above has detailed, step by step instructions on how to get over the border safely.
Visa procedure
If you're staying in Thailand longer than 15 days, you'll need to get a tourist visa in advance from the Thai embassy in Phhnom Penh, or fly in to be granted a 30 day visa exemption (UK citizens). It seems there is some confusion as to whether you get 30 or 15 days when entering overland, but to be safe we will say 15 day visa exemption when arriving overland.
At the immigration office make sure you get stamped out of Cambodia and then into Thailand, and pay the immigration fee (1000THB) if it exists (only found one mention of it).
Transport
Here we will be taking the easy way out, and saving a full days uncomfortable travelling by taking a 2 hour flight from Krabi to Georgetown for around £40 each. Cop-outs, I know.
Visa Procedure
British nationals don't need a visa to enter Malaysia, we are automatically allowed to stay for 3 months. So this may be the easiest border crossing yet.
Transport
Here we are travelling from Kuala Lumpur to Jakarta, a long overland and sea journey. The quickest, simplest and cheapest way to make this journey is to fly, with flights costing around £16.
Visa
UK citizens will be allowed entry on arrival for up to 30 days without a visa.
So our final border crossing is another easy one. :)
Transport
Buses from HCMC cost 13-18USD (or up to 35USD for sleeper buses) and takes 6 hours. Mekong Express is the most highly recommended company by other blog users.These buses stop at the border, but no where else, so stock up on your snacks and boredom breakers.
Visa Procedure
At the border you will be asked to get off the bus to sort out your visa, so have your passport photos at the ready! Purchase a Cambodian tourist visa (T-class) for 35USD (payment in USD only, they will not accept their own currency) get stamped out of Vietnam, and stamped into Cambodia, and get your bus the other side.
Georgetown to Ipoh
There are a number of bus companies covering this route, buses can be booked online or in Georgetown. The journey takes roughly 2 hours with a 20 minute toilet/leg stretch break. It's costs 15-25MYR depending on which bus company you use.
The old train takes about 4 hours and costs 12MYR, or the new electric train takes just over half the time at twice the cost. So, money or time?
Bangkok to Ko Pha Ngan
Most travel agents can sell you a bus/ferry combined overnight ticket to Ko Pha Ngan for around 600THB. The journey takes 12-14 hours. Alternatively you can take the government-run route, which is supposed to more comfortable, but will set you back 1000THB.
Ko Pha Ngan to Khao Sok
First we take the Lomprayah ferry to Surat Thani for 700THB. There are cheaper options, but this method takes less than half the travel time at 1.5 hours. There are ferries leaving Ko Pha Ngan at 7.00 and 12.00 daily.
Public buses run from 7.00 leaving Surat Thani every hour, on the hour, or from the train station at 15 minutes past the hour. The journey takes 2 hours and costs 120THB
Khao Sok to Krabi
There is no direct bus from Khao Sak to Krabi, (but this is information from 2013 so will check while I'm there too) so the easiest option is to take a minibus. These cost ~300THB and take 2-3 hours. Buses leave at 8.30, 10.00 and 12.00am. Make sure you ask the driver to drop you in the town centre, or you'll be dropped outside of town and have to make your way back in.
Krabi to Ko Jum to Ko Phi Phi
To get to Ko Jum you must catch a ferry from Krabi to Ko Lanta, and get dropped off halfway inbetween, where longtail boats will take you to the shores of Ko Jum. This should cost 400THB, pre-booking is recommended as it is often full.
There is a cheaper, albeit more complicated route, Catch a songthaew to Nua klong for 20 THB (though they may try to charge 50THB for tourists). Then catch another songthaew to Laem Kruat for 50 THB There are ferries leaving throughout the day from Laem Kruat to Ko Jum for ~70THB. It is a 40 minute boat ride.
There isn't much information on how to get from Ko Jum to Ko Phi Phi, but it seems there are direct ferries running from 8.30am, which can be booked through you guesthouse who will have to ring 081-968-9457 or 085-691-1857 (don't ask me why there's two numbers). I'm not sure on the price. but I will find out better once I'm there.
Phnom Penh to Battambang
There is actually a free option here. The old train is free, though it takes a little longer at 13-14 hours, but it is completely free, and a good chance to mingle with locals.
Or for a shorter route, you can take the bus for 24,000 riel and this will only take you 6-7 hours.
Battambang to Siem Reap
Again there are two options; bus or boat. The ferries leave at 7am, taking 6-9 hours, and cost 82000 riel, the bus is a shorter journey at 4-5 hours, and costs only 6250 riel. The boat journey is said to be far more scenic, and the bus journeys are far more frequent, so it's your choice depending on how much money/time you have.