Not even just 'jihadists'; Muslims full stop. Islam is more akin to a nationality and not just a 'religion', so having Muslims join the British military makes as much sense as letting (say) Russians join.
It's funny that the same British top brass who would be horrified to let Russians join think it's wonderful when our military becomes more 'diverse' and apparently 'stronger' by letting Muslims, women, trannies etc. join. Either they're dense (possibly) or deliberately undermining it.