Don't panic too much. London is usually pretty quick for UK citizens. You could order notes to see where they are in the process (I believe they take a month to get).
I'm wondering, how set in stone are these "no extension" rules? Do the officers still have discretion over these things in extenuating circumstances? I realise the rules are on the website, but that might not be the actual rules.
Only reason I ask is, back in the day, my husband's work permit was limited to 6 months, no extensions (at the time) and he still tried for an extension and it was granted.
Thoughts?