It would probably get too complicated, too many different categories, if there were such exceptions rather than just broadly making it by professed faith or official membership in the general religion. Besides which, the conversion may indicate that they might have a variant/unusual (for an indigenous Briton) personality better suited to Islam/Shariah. If not, make efforts to convert them back to Christianity (and criminalize with severe penatlies any crimes committed in Muslims punishing them for this).
Also, a more traditional Christianity needs to be encouraged. Most modern Western "Christianity" is of little use as a source of moral teachings and motivations. That's the reason for most conversions to Islam: people seeking what Christianity itself once provided, a moral framework and motivation to behave morally.
