I have a GSM phone. Is there any way to know if it will work in the UK?

- Is it quad band? If yes, it will work.

- Is it unlocked? If yes it will work.

If one or both the above is not true or unknown you can
pick up a cheap pay as you go phone when you get there.
There are phone shops on every street and often free SIM
cards being handed out.

UK uses 900/1800mhz, if you put the make and model of your phone in a search engine, you should be able to find a website (the manufacturers)that tells you if it operates on those frequencies. Usually best to get a UK sim card (sometimes free) as well.

What kind of service is attached to your SIM card?

My phone is an LG 620G. I noticed that it is 850/900/1800/1900 MHz but not sure if it is quad band or not.

<i> I noticed that it is 850/900/1800/1900 MHz but not sure if it is quad band or not.</i>

That's what "quad band" means.

"850/900/1800/1900 MHz" Four bands, yes it's quad (4) band.

Now you know that it's quad band you need to get it unlocked.
Look on Kijiji.com or craiglist.com for unlocking services. I paid $35 to have an iPhone unlocked recently.

I found a way online to unlock it. It is samsung t459g. Willl it work over there then and where would be the easiest place to get a SIM card after landing

I went by the local t-mobile store, and verified with them that the phone is unlocked. They put one of there sim cards in it(BTW, phone did come other carrier than t-mobile) and immediately the phone was able to make calls. They said I would not have any trouble getting any sim card to work over there. Yay!

Now to decide, since my connections are short, do I get a sim card from one of the vending machines at heathrow(anyone know if there is any in T3 and how much they cost), or along the tube somewhere

Look at http://www.lebara.co.uk They're very inexpensive.

Are you in the UK now? They are offering SIMs... fill in the form and have one delivered to where you'll be staying.