Hi Charlie,
a quick search on the internet showed that there are several foundries in your area that may be able to cast some blanks for you from the existing bases. "thebrassfoundry.co.uk" Is a small foundry that does mostly brass but could probably do aluminium for you. "investacast.com" does aluminium but seems to be a much larger operation that may not be as willing to help with a small job like yours.
I have used this approach both in England and the USA to get things I could not get any other way, including an 85 pound three part, fisherman style anchor I designed.
There may be more but I only looked in Cornwall and Devon.
All the best, David.