OP - we spent in total, including honeymoon, £20k 7 years ago. I don't regret it at all, however, we didn't have any DC at that point and easily afforded it (those were the days!).
In that, we only paid for the hotel rooms for ourselves, the best man and bridesmaid. Everyone else sorted themselves out or went home. I have been to a wedding or two where accomodation was paid for, but not in a hotel (more they hired a stately home with rooms and we got one) and that's 2 out of the 40+ weddings I've been to, including 4 in other countries.
You are being over generous offering to pay for all the rooms. But if you want to do that, then I think you need to change the date and venue so the costs are more reasonable.
If you want winter wedding, go for the end of January, it'll be a lot cheaper and you won't be clashing with guests already having plans. (But avoid February as prices go back up for Valentines day).
If you are going to put people up, give up on the Mill, and look for hotels, there are a lot of stately home hotel types that will let you do exclusive use, that means you will get the rooms at a massively discounted price - and effectively not pay venue hire as you are paying for B&B for everyone.
Alternatively again, go for a hotel type venue, book just hotel rooms for you and your main wedding party, then they will normally give a discount code for other people chosing to stay over - let your guests sort that out for themselves.
In my experience, those lovely Mill type venues and up being the most expensive weddings, hotels are set up with catering on site, hotel rooms on site and you are effectively getting a discount for booking several elements together from the same supplier (the hotel), even if the 'package' cost looks very high compared to just the venue hire for places like Mills/large houses/barns/museums. As you've started to realise, the extras like linen and corkage, bar staff etc add up. That's 'all in' for most hotel deals, or at least can be haggled to be included compared to having ot get that externally.
Back to the drawing board. If you say were you are in the country and rough numbers, I bet we could find you venues to give you a real "wow!" day for well under a £15k budget for end of Jan 2016.