Pros and cons to buying daytrip train ticket in advance?

Here's a quote from the National Rail site about Advance fare tickets: "Advance fares are valid only on the date and train shown on the ticket and are non-refundable. You can change the time or date of travel before departure of the first reserved train. Any difference in fare and an administration fee will be payable."

Two years ago I was working for a couple of weeks outside Ely. When the job was finished I wanted to go to York for four days before flying home from London Heathrow. I bought a one-way ticket from Ely to York and made the trip. While in York I met a person who told me I could save money by purchasing my return ticket in advance. I went to York station and bought a York to London ticket for two days later for only about 40% of what my earlier Ely to York ticket cost. Yes, even for day trips advance purchasing saves money!

It's pretty simple. The disadvantage is loss of flexibility (weather, missing your train, etc). The advantage is saving a ton of money. If you can soldier on through bad weather, don't mind being locked into a schedule, and can make it to the station on time, buying in advance is a great deal.

Thanks for the advice, everyone. I guess I'm hesitating not only because of the weather possibility but also because I'm hoping some friends can visit me in London and I don't know when they will come, if they'll come. It's tempting to buy tickets in advance to save money, but I don't know how much I'll be saving. I will probably know my friends' schedule by the time I get to London so I'm thinking of waiting until I get to the city before I buy tickets. I know no one can tell me what to do, and I have to decide if saving some money is worth an inflexible date. I just wanted to run this question by you guys in case you mentioned things I hadn't thought of. Thanks again!