I've got 3 lads working for me now, 2 trainees and 1 experienced plumber who can also fit bathrooms and kitchens for me.
One of the trainees is older and being trained up by me to do electrical and gas work. The other is an 18 year old apprentice who has just finished his first year.
I did try sub contracting work out but from my experience: -
1.) There's very little profit left in a job after paying subcontracting rates for someone else to come in.
2.) Most subcontractors will always prioritise their work over yours and end up stiffing you at some point. Not great if this happens half way through a bathroom installation they are doing for you.
When you employ staff you have more control, this is the big advantage for me. An employee should care much more about the work they do for you than a subcontractor. That's been my experience anyway.
This said, I've had some awful employees in my time. The good news is that all employees are basically on a 2 year probation period with you, despite what you may set formally. If they are crap in this time and you sack them, they have no right at all to take you to an employment tribunal (exception - if you sack them because of their gender, sexual orientation or race).
We recruited our bathroom fitter through a free job centre advert.
https://www.gov.uk/advertise-job
Do be aware that plenty of people will come and offer to work for you despite not having the experience you ask for. We had one awful experience where I took someone on who claimed to tile. I sat about while they started. After 2 hours he had put 2 tiles on the floor. He was awful, even my first year apprentice knew how to tile better than he did.
As long as you closely supervise anyone you take on until you know you can trust them you should be ok.
Keeping track of van stock etc is something worth bearing in mind. We have a stock control program we've had designed ourselves specifically for the plumbing industry. It allows me to track stock levels for all of our stock items in any van. I can do this from my PC in the office or using my smart phone. Stock is scanned in and out from the vans using the lads smart phones and QR codes. This allows us to automatically order new stock for the vans as required. One thing I didn't really think about before expanding to a 2 van business. It also allows us to keep a close eye on what is being used and to make sure no one is taking stock without permission!
Finally, remember that if you sub-contract a lot of work to one person they will be considered as employed by HMRC.
Hope some of this is useful and helps.