You can upgrade from the Max HD package to the Max 4K and then pay £99 plus postage for the upgraded BTTV box, BT have already provided one free, any more or upgrades are chargeable but at a reduced rate, they retail for £299.
Might be worthwhile calling our Loyalty team on 0800800150, if you choose to upgrade your package it may be possible to get the box. Alternatively have you tried to upgrade your tv package on bt.com?
Pricing has changed you can now as part of an upgrade order get an Ultra HD box for £49 with a postage charge of £9.99. Alternatively you can buy one outright for £249.