It could be well possible that I have covered your area in constructing a cake making database though it isn't yet complete.....if you are in the south or northern England then I have covered every home and small business.
I think the problem here is that those sites quoted are likely for a standard wedding cake mix of rich fruit and royal icing. A coffee and walnut cake is totally different, try a few local to yourself for quotes.
It would depend on how many you want to feed and what decorations you would like on it. My daughter is a registered cake maker from home. She spends time talking over designs, drawing them, pricing each item. It takes time and a lot if skill. Which is why cakes can be expensive, try asking round your local area, about home bakers, if you have facebook a lot advertise on there and that way you can see their work.