most supermarkets sell vegan alternatives to milk for relatively cheap (£1-£2), i'm not sure about soy milk in particular, but I really recommend unsweetened almond milk. Another option is to make your own vegan milk! If you have a food processor all you have to do is soak some oats in water over night and then sieve the pulp in the morning and the liquid is your oat milk... simple and very cheap! Also, if you want the oat milk to be a little sweeter, add a tiny bit of maple syrup or something to the original soaking oats...
I would also suggest trying out nutritional yeast (£2.00) as a replacement for cheese (vegan or not) because it gives you your B12 vits which are sooo important and also tastes great and the pot will last you at least 4 weeks depending how much you use it.
Alpro soya yogurts are a great vegan alternative to yogurt and they taste AMAZING! they are all accessible from ALL big supermarkets like tesco etc. I find that when I shop at ALDI or LIDL I just stick to getting my fruits veg and pulses etc then get the more specific stuff from tescos.
Hope this helps and if you have any other questions about veganism just ask!! x