Weeell, it's not just breast cancer that spaying reduces the risk of (although it does dramatically reduce that risk). There are other tumours an intact female is at risk of, and also pyometra to worry about with every heat cycle... The costs of treatment for any of those diseases would run into the thousands, and can still kill. Pets are expensive - that's just how it is, and we all know it before we buy.
As for low-cost or subsidised schemes in the UK, yes they exist. Check with your vet office to see if there are any schemes running in your area (Note that these are likely to be restricted to those unable to cover the cost of spaying their pets, not those who don't want to). I know there are pamphlets about that in my vet's office...
You could also do a search online. Here is one link that I found, no doubt there are others:
http://www.friendsoftheanimals.co.uk/default2.htm
And this one, which is actually about cats, but gives a list of low-cost spay/neuter clinics - I'm sure they do dogs too:
http://www.catchat.org/neuter.html#lowcost
I believe you can also ask your local RSPCA for vouchers to assist with the cost of spaying/neutering.
Incidentally, if you're not already aware, do ensure that whatever vet you use knows that boxers cannot be given Acepromazine. Ace is a commonly used pre-anasthetic sedative, but it is known to cause heart failure in boxers. If a sedative is required, an alternative such as valium should be used on this breed.
