I'm looking at options for internal wall insulation. The wall buildup will be solid brick wall-battens-insulation-VCL-battens-plasterboard.

The obvious choices for the insulation are foil-faced PIR, eg Celotex GA4000, foil-faced phenolic, eg Kingspan K12, or foil-faced PUR eg Kingspan TW55.

I was interested in looking at XPS as an alternative because it seems to be considered more eco-friendly than the above, doesn't seem to have the shrinkage worries, and is potentially cheaper (?). But I can't seem to find anyone selling a foil-faced version for walls.

Is that because such a thing doesn't exist?

The obvious question is, why do I want a foil-faced board - well my understanding is that it improves performance and provides a surface that can be tape-jointed for vapour and airtightness barrier - presumably if I went with simple XPS boards I'd have to do the vapour/air barrier with a separate sheet of polythene instead?

Lineweight, you can purchase XPS ready bonded to plasterboard. The brands Styroliner or Plusboard from Panel Systems and Proctors respectively. Neither typically incorporates a foil VCL as the vapour resistivity of the XPS (Styrofoam in these two cases) is so high. Care is needed at the joints but as a belt and braces vapour control installation two coats of drywall topcoat on the fixed boards will suffice

Posted By: SaintLineweight, you can purchase XPS ready bonded to plasterboard. The brands Styroliner or Plusboard from Panel Systems and Proctors respectively. Neither typically incorporates a foil VCL as the vapour resistivity of the XPS (Styrofoam in these two cases) is so high. Care is needed at the joints but as a belt and braces vapour control installation two coats of drywall topcoat on the fixed boards will suffice

Sure, but for various reasons I've decided I don't want to go for a bonded plasterboard/insulation arrangement.

I'd go with Timber's suggestion then in that case a separate VCL or even an AVCL
With XPS and EPS its another operation to laminate a foil whereas for the thermosets i.e PUR, PIR etc the foil facings come as part of the manufacturing process.
You'd have to go to a 3rd party laminator or stockist with a laminating facility. Maybe Encon. Foil would add little thermally if anything to EPS or XPS and if they were available with one side foiled you can bet that someone would install it the wrong way round

Same for PIR/PUR