You will definitely be operating commercially so you do need to get the qualifications.
Read this, it lays out pretty much everything you need to know.
http://www.caa.co.uk/docs/33/CAP%207...rch%202015.pdf
Look at chapter 4 regarding competency requirements. PPL experience does count and you may be able to miss the normal ground school element and only do the flight test.
Talk to the guys at Resource Group about courses, they are the most professional company in the UK.
Unmanned Aviation Services Training | Resource Group
For insurance, talk to
https://www.johnheath.com/business-i...uav-insurance/
Operating in congested areas opens up a whole new can of worms and is best left to the professionals.
For small jobs its worth finding a local qualified operator and using them may well work out cheaper.