Yes that is actually quite small. But you should be able to fit in: a few home grown tomatoes (the real reason to have an allotment! - just pick up 2 or 3 plants now, too late to sow your own), some lettuce, a row of carrots (may or may not have luck with those!), French beans definitely, they usually grow very easily, and you can still now sow beetroot, parsnip, broccoli (Calabrese), and you can sow now seeds for the autumn like kale, brussel sprouts, etc.
If you want some strawberries, you don't have to plant those till autumn and you can have your first crop next year.
have fun! I would say a couple of visits a week is sufficient for 25 sq m. I work on 100 and I spend about 3 or 4 hours a week, would be better if it were 6 but I get by.