Extend at same day appointment, travel without biometric?

[nfoote]

So I'm extending my Tier 1 (General) visa for the 2nd time.

I have a same day appointment booked for the 17th and will be paying the extra £££ for this service at which I'll get my passport back on the same day.

I'd like to travel to Germany on the 20th, returning on the 23rd.

My current biometric permit expires on the 22nd.

I understand that my new biometric permit will be posted to me AFTER my same day appointment. So unless it arrives on the 18/19/20th I won't have it until after I return from my trip.

Do I neeeeeeed it to come back into the UK?
Alternatively, could I not just re-enter the UK as a tourist (which I've never done before, always had a Tier 1 General Visa with me) and get my biometric permit (ie work visa) when I get home?

Or am I stuffed and can't book my trip until the very last minute IF the permit arrives in the 2 day window before the trip?