Evening Reception-only Wedding (uk)

Hi all, has anyone had any experience in planning a reception-only wedding? My fiance and I are toying with the idea of having a small wedding ceremony in a registry office with close friends and family and then arranging a big party for later in the evening. However, we have been looking at venues (in Essex where we are based) and we are not sure how to discuss this with venues. By this I mean, it won't be a wedding, but just a party to celebrate the wedding. We are concerned that venues will use this as an excuse to charge more than what they may charge for a party (i.e. engagement, anniversary etc). Most venues want you to enquire and specify the "type" of party that you want to host. Essentially, can anyone provide any tips and tricks to avoid being unnecessarily caught out? Thanks