i think to clarify some of the above comments:

- you can apply a GUF to a booking made using a companion voucher
- you cannot stack GUFs, or UuAs, or apply a GUF to a UuA'ed booking - or vice versa
- you cannot combine barclays vouchers with GUFs

in terms of making a reward booking using a GUF online, it only seem to offer you one price point - usually the highest avios one which personally i don't find to be good value. instead, you can make the reward booking online for the un-upgraded flight and it should offer all the pricing points which you can pick from, then call to apply the GUF - albeit this method would require reward availability in the lower cabin.