I've been into Elstree a few times. It seems they want proof of purchase rather than warranty cards. If your receipts are from a UK supplier and show VAT having been paid then you should have no problem getting the stuff sorted under warranty. Lenses have an international warranty, but bodies do not.
I've had lenses and bodies calibrated and repaired by Elstree. Many were within warranty and completely free of charge. I had some lenses calibrated outside warranty but I can't remember the bill for those. It was, I think, about £30 per lens whether a simple prime like a 50/1.4 or a more complex zoom like the 70-200/2.8L IS.
BUT, before going down that path you might want to check that your problem is one of poor calibration rather than simply a duff lens. You may have already done that, but I haven't spotted a post anywhere where you mention that.
To check the sharpness of the lens when focused properly use a tripod and Live view and focus on a well lit, contrasty target using Live AF. You might also try manual AF using 10X magnification through Live View. If you can get sharp results like that, but when using Quick AF the image is soft then that would suggest poor AF calibration. However, if everything is soft then that would suggest duff lens optics as the primary problem. You might have an AF calibration problem too, but if the lens is basically genuinely soft, rather than simply misfocused, I think that needs dealing with first.