Answers
1) You can work without plans, but how do you know what the builder is going to provide you with? You won't be able to compare quotes, like-for-like, without plans, and some accompanying spec. details about other parts of your requirements. Bit of a generalisation, but I would tend to be more trusting of a builder who says he would like to see your plans upfront to quote from - those who are willing to work without are perhaps more likely to say 'that'll be a extra grand if you want it like that'. They'll do that half-way through the job.
2) Builders will check for where the pipes are.
3) You need Building Regs and there are 2 ways to do it - with plans or without. With plans, one registers the plans with Building Control (this is a differenr department to Planning) and they come out periodically to check what the builder has done. Or you can use a private building control chappie. Alternatively you work without plans and Building Control come and approve what they see (or not). Better to have plans IMHO.
Most builders can arrange this.