Internship for 6 months - Problem with Expiry

Hello community,
Following situation:
I am going to start an internship in LONDON on 11.9.17 until 28.02.2018 am and I need a visa (Tier 5). I have Turkish citizenship with permanent residence. Unfortunately this year the validity of my passport runs out. (30.12.2017 = Date of Expiry)

My question:
Should I extend my passport? (Duration 2 weeks) And apply then for a visa?

OR

Or is it "OK" with my expiration date? Because you have 90 days time to leave the country.

I have Turkish citizenship with permanent residence.

What do you mean by this? Which country do you have Permanent Residence?

You should renew your passport before you travel otherwise your visa will likely only be issued up the date your passport expires.

In Germany I have Permanent Residence.

How long it takes to get a Visa and What forms do I need?