Emma Dovey wrote:
I understand that transcription into another language is [paid] at a higher rate than monolingual transcription...
Most agencies asking for transcription will want to pay a lot less than what may be worth your while. Some such agencies expect to work with translators who are happy to earn no more than about EUR 10 per actual hour.
Although it's possible to translate while you listen, typically it's easier to just transcribe in the audio language, and then translate your transcription. This means that it would make most sense to charge one rate for the transcription and another rate for the translation, but often such agencies will want you to charge a single, unified rate for the service as a whole.
It takes 5-10 minutes of actual time to transcribe 1 minute of audio. This means that when clients ask for your per-minute transcription rate, you should take your usual hourly rate, divide it by 60, and multiply by between 5 and 10.
Strictly speaking, it takes longer to translate transcribed text than to translate text that was originally written with translation in mind, or text that was written with publication or distribution in mind. Spoken text typically isn't already suited for translation, so when you translate such content, you have a harder time figuring out how to best translate it. But clients will expect you to charge the same (or even less) than for normal translation.
I'm also UK based and would prefer rates in GBP, other forums I've looked at seem to suggest varying rates but in USD but I don't know if simply converting the currency would be appropriate.
UK rates (at least for translation) tend to be lower than non-UK rates, and the UK currency has lost a lot of value over the past decade, so feel free to simply convert using the current exchange rate.
[Note: I'm not a transcriptionist myself, but I'm often offered such jobs.]