Firstly, if you decide to stick with DIY, make sure you can go to the place the day before to set up, and the day after to take everything down, because you definitely DON'T want to be doing that on your wedding day!
We're doing a bit of a combination - we've booked a venue where we have free rein to organise our own bar/caters/decor etc, but the guy who runs it will also help with the set up, and will assist our suppliers on the day so that we don't need to think about it. Maybe look around for something like that? Most venues seem to have some sort of coordinator person who will assist as much as you require them, which means you get the help without having to pay out for a dedicated wedding planner.
Budget-wise, unfortunately, whilst your quotes are a little higher than what I've had (I'm in Somerset), I wouldn't say they were totally shocking, considering your proximity to London. The only thing you can do is keep looking, and maybe try negotiating what you get for your money. If you try and book something soon, you might be able to get it for 2018 prices, or you could try going to wedding fairs - especially when they are held at venues you like - as they often give a discount.
Or, if you don't have a date fixed, you could wait and try to get a last minute deal from a cancellation, but obviously there are no guarantees with that.
Some friends of my chap got married last year in Sussex, and it was a nice sized venue for the numbers your are talking about - I'll try and find out the name for you - have no idea on prices though I'm afraid!
Try not to panic, it will all work out in the end!