Hi Liam,
Welcome to the Forum and site.
If the plot you have is ready to go and you are quick you could still get a sowing of French Beans, Beetroot and spinach. You can still sow lettuce, Radish and Spring Onions. Start looking for Japanese Onion sets as they can go in next month for harvesting next year. You might also like to try some leeks for over wintering. if you have a cold frame or greenhouse you could start some potatoes in a large container for Christmas, but they need to be keep well out of the way of frost.
I would suggest having a chat to some of your fellow plot holders for advice on this list as I am in the south of England and you are a little further north.
Have a look at my gallery as I have posted some pictures today of my plot earlier this year and how it looks now. But whatever you do keep coming back to this site as it offers a lot of friendly advice and encouragement. Owning an allotment is all about having fun and enjoying what you produce.
Once again welcome
Rugbymad40