I've been to both previous fixtures and am going this weekend howecer I travel as part of the club. Flights are expensive and I don't think there is much difference in prices across airlines. The cheapest option is the ferry. I know you can travel from Weymouth which is obviously a long car journey but I'm not sure if there's a closer option ie Portsmouth. We are traveling there and back in a day however there are various hotels, St Pierre I believe it is called and is very nice. It is just around the corner from the stadium and an easy walk if you don't have any difficulties in walking. I know a few of our supporters travel out and stay for a few days so hotel advice from them may be a better option.
As Lenny says flights can be expensive. You can get details if you look up the Aurigny Web Site. Aurigny specialise in flights to the Channel Islands. The earlier you book the cheaper it is. The ground is about a mile from St.Peter Port and there are plenty of Hotels and Guest Houses to allow for every budget. There are plenty of cafes and restaurants but you won't find a McDonalds or KFC. Buses are frequent and there appears to be a standard charge of £1 to anywhere, even a trip round the island. We stay at the St.Pierre Park hotel which is a five minute walk to the ground.
One thing to stress ,if you are going, is that Guernsey is not part of the NHS. So you either take a chance or get some insurance. I was told that to call a Doctor is £47 and An Ambulance is £120.
Just to add to Philss reference to The NHS, you can get some very good deals for insurance online. I have a less than favourable history of health and got insurance for about £5 last season, I suggest you try the comparison websites and make your choice from there.