I'm at Center Parcs Longleat at this moment in time. I wouldn't say it is overpriced, if going out of school holiday time. I'm in a 3 bedroom Woodland villa and it is costing £50 per night - it can sleep 6 people.
If your 12 month old likes water, likes walks in the woods, then I'd say it's fine for them. If you like Sauna's - then Longleat Spa is great (though your DH/DP will need to care for your DD), or you will need to use the childcare facility - no babies allowed in the Spa.
Babies don't need things to do. They like to watch others... most babies I know are happy to people watch, go for a crawl around, play with their toys. Some babies love playing in water... some don't. With a baby, going to Centre Parcs isn't for the babies benefit... it's for yours.
Have just posted on another thread with regard to my (and my mum, who I've taken on my trip) experience today, eating out. Not up to usual Center Parcs standard I'm afraid to say... may even need to have words with CP Customer Services - their concessions have let them down. With luck it is a glitch - I was here in August and October last year and on both occasions there were no issues (though didn't use those particular restaurants on those visits). So I hope it's just a glitch, rather than a new trend.