Galvanizers association lists all the hot dip galvanizers in your area.
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providing you can find a galvaniser who has the necessary equipment to a) clean the chain and b) agitate the chain during galvanising. They are in short supply and only about 1 in 10 can do the job.
I confess I've found it cheaper to go to the UK manufacturers and buy new at trade prices and get calibrated, certificated chain (which is quite different to the expensive stuff on offer in marine chandlers)
I forgot to mention about minimum charges - unless you've about 80m of 8mm, you'll find it uneconomic unless you can put it into a load with someone elses' galvanising. Minimum weights are now about 100 kg tho' one I know locally will do a minimum of 25kg.
Much easier, sharpen, get it shot blasted and then put in with someone elses' galvanising load. there are any number of small engineering shops who send stuff off to galvanise.
PS I'm assuming a plough type anchor here, I do mine every 3 - 4 years. Watch out if the shank is cast, galvanising can cause embrittlement.
They sent a combined load off a year or so ago. My bit was just a 35lb CQR which came back as good as new. Total cost (work, carriage VAT et al) was about £26. I don't know if they're doing the same again this year but it could be worth a call.