Do you ever visit back home in Wales? I live up in the Pennines and my parents lived in Sussex. Going 'home' was like travelling in time, you could go forward a whole season one way and go back one on the return journey. I would say Liri is right, at least 2 weeks and probably nearer 3, though it may vary slightly from year to year depending on the pattern of weather distribution. I always look at the trees on Boat race weekend, or when they do TV interviews from Westminster just to see how far ahead things are down there!
You need to employ more tricks to lenghten the growing season: black plastic and cloches to warm the ground , extra fleece for frost and cold protection, maybe windbreak netting if necessary, to reduce damage and wind chill and to provide a more favourable environment for pollinating insects. One year we had a warm April followed by a cold May and my broad bean plants stood covered in flowers for 3 weeks, with scarcely a bean to show for all their effort!
Start your plants off under cover to bring them on as far as you can, so that when spring does finally arrive you have stronger plants to get you going and don't have to wait for uncertain germination. Use modules or grow rows of peas in guttering so they can hit the round running, so to speak!