I have numbers that I pass work on to when I'm too busy to take it on. I trust them not to let me or the customer down so its a small handful of folk. To answer the question though, I personally do all trades bar Gas work who I sub out to one man only.
Same here.
If you're just starting out leave it to your customer to organise the other trades. The last thing you want to do is be associated with someone you've put forward and who isn't worth a bean.
Get involved with what you need the other tradesman to achieve so you can do your job. After a few years of working in the trade you'll build an address book of trades you would 1) recommend because you've worked with, and seen their work so therefore can recommend and 2) are happy to work in conjunction with.
It is possible to be good at your job but a nightmare to work with. A bad relationship between your recommended tradesman and you customer will have a negative effect on your customers relationship with you and therefore any future recommendations they may make for you. If you wanna be successful protect your own reputation.