I believe that when you are registered as self-employed, you need to be able to demonstrate that your income is obtained from multiple sources and that none of these sources is heavily over-represented. It's a bit hard to explain and I'll probably need to check the current guidelines.
In my case, I am self-employed providing professional services on an irregular basis (and no, not that kind of service if that's what you're thinking
). I work as a locum, but I have to be careful not to work for any employer too often or the HMRC can accuse them of avoiding NI and tax contributions by not putting me on their payroll. I try to spread myself around so that I'm invoicing several employers, and as I'm personally paying NI and tax the revenue isn't missing out.
I wonder whether issues could arise with any of the bigger home-earning opportunities? Eg if you did a lot of work for 63336 or Lionbridge, or even spent the whole day on Swagbucks, and they were your main source of income - could the HMRC get fidgety do you think? Maybe if you can demonstrate that you are bringing in various forms of income (even in the form of vouchers) from a range of survey/PTC sources etc, this could work in your favour? But I've not thought about it that deeply to be honest