We took DS for his birthday last month and it was, hands down, the most expensive place we'd ever been. My DGran gave me £100 for us to have a day out and that was completely gone and then some after train tickets, entry tickets and a guidebook. We finished up having to go and get our lunch in a pub in Camden because we couldn't afford to eat at the zoo!
If you are going to visit definitely more than once in a year then it's probably worth getting the annual membership. Can you get tickets with Clubcard vouchers?
Can you not even just pay for one train ticket for one stop and use that? I don't think you even need to use the train technically. And at 2 for 1 buying a rail ticket for £4 or something makes it worth it imo.
We're also thinking of visiting next Saturday - I have downloaded the national rail voucher as last time we used one for the aquarium our train tickets didn't even get checked. Or you could buy in advance. Either way, I've heard some mixed reviews for London Zoo - we're travelling quite a distance and have a 3.5 and a 1yr old. Is it worth it? I have read/been told that there is not always that much to see - the animals are not that obliging!!!
Definitely buy tickets on line , they don't sell family tickets on the day... Cue my rant at the ticket office. There is a free three months friends and family railcard voucher in the daily mail today, but may not be in time for your trip, worth a look
Daysoutguide.co.uk download the 2 for one offer. Get thee to the nearest train station in London, get a vauxhall to Waterloo ticket (one way approx £2:50) and show with the voucher at gate. We did this exact thing last June, it worked like a dream. Make sure the train ticket has the birds feet type sigil on it, not the London underground.