As above. You can get married in any council. Eg: City of Westminster if you want a wedding in central London. They have a list of townhalls and nice period style buildings.
Horniman gardens is a premium venue. Costs a fortune and you have to pay for caterers to come in etc. As well as decorators the whole lot.
There was a wedding at Horniman featured in some magazine last year. Had fireworks and everything. They spent £20,000 on catering alone. I'm sure you can do it cheaper but that's the kind of people that have their wedding there.