Just to restate Terry – whether you need any testing/certification at all, really depends on where you want to use your boat. At home, you need no tests/certificates to run your boat at all. However you are still the person who has to decide if the device is operating (and being operated) within its safe limits – and even a low pressure boiler can present some risks if used incorrectly (by an unsupervised child for instance).
You mentioned that your boiler already has a safety valve fitted and as there is no requirement to fit a second that I'm aware of, I'm not sure why you are concerned about this. It does make sense to check that any safety valve that is fitted, is operating normally, even if you do not have the means to check it exactly against a calibrated gauge.
BTW – the code states the following with regards to "new" commercial boilers:
"It does not apply to Commercially Built boilers which have been either manufacturer fitted to a complete model, or have been installed in a home built model, and were issued at manufacture with certificate of conformance with sound engineering practice (SEP) as outlined in the Pressure Equipment (Safety) Regulations, unless that boiler has received structural repair or alteration"
And whilst it should be safe to assume that any well known brand does conform to SEP – I'm not sure I would just assume that every "commercial" boiler has been tested to a higher/better level than that required for "home-made" boilers – as frankly, it may not have been. It's also very easy to purchase all sorts of stuff from abroad/eBay these days – so a little caution might be useful..
Regards,
IanT