Depending what you mean by "quite a large cap",
IDNet might suit you, either the Home Max or Home 2000 services. Obviously, interleave wouldn't apply to the Home 2000 service, but it's not clear from your post if your line will support this fixed speed service.
IDNet default interleave to off, if you're concerned about keeping it that way give their CS a ring. It hasn't been a problem to me so I can't give you a definitive answer.
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Bill
IDNet with MaxDSL
If you want a vision of the future, imagine a boot stamping on a human face - forever.
George Orwell