Oh, come off it... single v double glazing in historic buildings isn't going to make much difference. Secondary glazing and draughtproofing all can also be considered. DG will eventually need replacing, when it fails.
The green solution of course would be to retain the uPVC windows, not replace, until they fall apart.
From the Jersey link I posted:
In most cases where old windows have not become totally beyond repair through lack of maintenance or very heavy use, recent local research has demonstrated that repair, overhaul and upgrading can be significantly cheaper than replacement in alternative materials, whilst providing a broadly similar level of performance (see Section 2 of Technical Information Pack).
The new Part L regs, which came into force in April, are meant to be interpreted on a 'whole house' basis. However, to date the only documentation available to building control officers is the official 'approved document'. and that tends to be followed to the letter by BCOs, even though it is not a legally binding document.
Unfortunately, I fear you may be on difficult ground iin challenging the regs to install single-glazed units (however historically and aesthetically necessary) in place of uPVC double-glazed units. Even under the previous Part L, this was a non-starter, and thus a precedent exists for compelling you to install DG units.
English Heritage is in the process of producing a new secondary document to help BCOs interpret Part L, but its earlier and excellent Building Regulations and Historic Buildings which detailed ways of installing secondary glazing to keep historic windows was aimed at listed buildings or those in conservation areas, and many BCOs are aware of this.
And, sadly, many of them seem to be all too keen to insist on the use of modern materials and some even seem to have a contempt for conservation. Grr!