I was going to update to the HH5 as it said here HH5 for £45 + £6.95 delivery but on clicking the link to order jumped to £75.95. A bit of a con to advertise it at £45 plus delivery then add £25 to the price on the order page. Even if VAT is added after it is only £9 not the £25 that they expect people to pay.
Check out the forum for the number of posts from people who have had problems with the Homehub 5. You would be better spending your money on a third party router.
You can get better dual band wireless routers for about the same money as you have quoted. You would still need to use your Openreach modem with them unless you decided to spend a bit more and get a VDSLmodem/router
the new hh5b may well be better which you should be able to get now
not everyone has a problem with the 5a - mine is ok and connected 36 days
BT charge different prices for the Hub5, depending on whether or not you're a customer. Perhaps the system didn't recognise you as a customer and asked the full price.
I must say that I have not noticed any real difference between the 5A and the 5B. However, others have said that there are fewer dropouts with the 5B. But, as I say, I have not noticed this myself after using a 5B for about a month now.