How can I get a consent letter for my visa application if I'm currently in Germany after applying from the UK?

I seen a couple instances where an applicant applied in one country, visited another during processing, and then went on to Thailand after approval. But these circumstances are rare and Embassy dependent. You would need to ask your particular Embassy which is processing the Visa if they will allow it. Think of it this way, in the old days you would have handed in your passport and wouldn't be able to travel. The consent declaration makes it pretty clear that they don't want you to leave the country, it's 10,000 baht you'll risk.