Hmm, I didn't read it as he's ordered to go! Biggles definitely has the choice to go! Raymond tells him so. He basically says "either you go or I'll send someone else, but I'm giving you the choice of going":

Biggles considered his chief with a wry smile. ‘I don’t think I’ve ever said this before, sir, but don’t you think, in asking me to take this on, you’re asking too much?’
‘Of course I do. I wouldn’t have asked you on my own account. Unfortunately the matter has come before a higher authority who regard the project as necessary in the national interest. There’s no compulsion about it. You are at liberty to decline. But if you don’t go someone else will have to be found who will go. I put the proposition to you, first because you’re officially employed here, and secondly, in the light of your experience, you’re better fitted than anyone else I know to tackle the job and pull it off.’
‘If you put it like that you make it difficult for me to refuse,’ muttered Biggles.
‘I thought you’d see it like that,’ murmured the Air Commodore.

Biggles: Well if Erich is getting rescued anyway it should be me, because I'll do it correctly -- with no strings attached, damnit.

I'm convinced he thinks that Erich doesn't want anything to do with them even in prison. BIGGLES!

Also there's a whole bunch of defeatism that is atypical for him that fascinates me:

‘But Sakhalin!’ protested Biggles. ‘The thing isn’t possible.’
‘It’s not like you to talk like that.’
‘I’m talking like it now.’

It's a fascinating book, I love to think about it. There needs to be approximately a bazillion of fic around this time period.