my (presumably) virginal and inexperienced young sister is left on her own with a horny and experienced gang-member.

I think he's assuming that the Thuggery don't look on Lawrie as female (see the passage early on in which Jukie non-verbally dismisses from consideration, from her pov): he's not to know that she's got herself up like a swinging chick, or that the Thuggery member designated to keep an eye on her is Mr Lech.

I can see that he might be happier not feeling that he had to go into the Navy - when prestonuk did her ODNB entry for Nicola as First Sea Lady, I wanted to have a coda mentioning Peter as a celebrated wildlife photographer. And Selby always sounds to me like one of those quietly quirky types.