I've done some research on this as I am developing a product to bring onto the UK market.
> What safety testing is required?
http://www.legislation.gov.uk/uksi/2016/1101/pdfs/uksi_20161101_en.pdf defines what is required. It's a superset of EU regs but basically you will need to self-certify it as CE approved. Start here, do lots of research:
https://www.gov.uk/guidance/ce-marking. Sometimes it's better to pay someone else to do this so you can point at them if it's wrong or sue them.
> Do I need EMC testing If I use a linear / transformer based PSU design (presumably I would if I designed SMPS)
Yes. EMC directive covers this. This is enforced by Ofcom and trading standards. See:
https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32014L0030 ... strictly you don't need to do this and can defer it but if Ofcom are called out or investigate a product of yours which has a negative EMC situation then you're up shit creek and are going to court.
> Should I have product liability insurance? - if so, how long is this required for, if someone is killed by my product 20 years down the line am I still liable?
Hell yes you definitely need insurance. See later.
> What is the approx cost of getting a home built product into a sell-able state?
Depends how much you do yourself. CE cert is paperwork and marking. EMC will most likely require a third party and cost ££££.
> Could I draw up a disclaimer to reduce my liability in the event of injury?
See later
> Could selling the tube amps as a 'parts kit' be a way around all this?
Yes the EMC directive and CE legislation only applies to finished products. This is a common workaround!
>Should I setup a limited company or similar to start selling these?
Definitely a limited company.
Key point: You need abstraction of liability even if you're doing face painting these days. Look up public liability insurance. Hide behind a limited company. It's not a perfect layer of protection but if you screw up then it seriously limits the damages, not that you intend to screw up based on these questions which are all sensible and good to hear being asked. Disclaimer doesn't really supercede the law. Be clear how the product is intended to be used in documentation and you are covered though if someone does something stupid later on. It won't stop someone suing you but it'll stop them from being successful. You would still have to pay to defend yourself so make sure all paperwork is in order.
Low voltage stuff is probably easiest to abstract responsibility away. Sell it with a CE approved wall wart as a self assembly kit and you're done.
Tubes and mains. Pfft I'd run a mile these days.