I don't know how much the Guildhall costs but it is a nice venue and should accommodate that number. The Grand Hall at St Martin's House (next to the cathedral) is also nice. Not sure about costs though.
Do you want to get married or do you want a wedding? IMO it would be crazy to spend your house deposit on just one day. You don't need 150 guests to get married. You could have a much smaller, cheaper wedding and still keep saving for a house - you have the rest of your married life together for big parties
thank you for those suggestions. In reponse to your question we want to get married without spending ridiculous amounts of money- i compared it to our deposit because they are the saving we have, we would both prefer to use the money towards a house as apposed to towards a wedding, but at the same due to family number it isn't possible to have a wedding with less guests than previously specified.