I usually start with $25 or $50 test campaign if an adnetwork does not have a minimum credit requirement. Also, i set enough budget for a day and use GA tracking link to track performance.
Every network has different minimums which usually reflect the type of clients they are targeting. Where I work at (PremiumCPM) we have a min. of $1,500 other networks like Adorika have a $5,000 min.
I'd deposit the lowest you can do, or at least £5 ($10ish). this can help you plan your program with your network..
I would kinda disagree with the part about starting small. Yes you want to start off "smaller" / lower than you normally would, but you need enough volume and enough of a budget to get a stable amount of data.