sdorling the Gold Upgrade for 2 - when applied to a revenue ticket - books into a revenue fare class. So, if you get a good T class deal you would then be booked into either I or D (can't remember which) for Club. While those are both restricted fare buckets and won't have much availability on a Friday night from SIN to LHR, you should be able to construct something.

Note that if applying the voucher to miles you only seem to get booked into the corresponding miles bucket in Club which narrows the options considerably.