Had a quick look at Xilo, they advertise an unlimited, unthrottled service for £18 a month, seems to have good user reviews?
The Xilo £17.99 package that you mention (Pro 16 Home) is based on the C&W network, so if you only have BT or CPW and not C&W, then you will not be able to get that one.
That being said, they do have a package, Home 5GB, which is £17.35/month, and comes with a 5GB peak usage allowance; with unlimited (200GB FUP) which is unthrottled. Off-peak is 12am - 8am M-F and all day Sat & Sun.
I'm currently with Xilo, and never had anything throttled on their network at all.