You should do a full duration test once a year to make sure the luminaire will run for its full 3 hours (or rated duration). You don't need to be an electrician to do this. It's best to stagger your testing over a period of time rather than trying to test everything at once. Sods law could kick in and you might have a power failure just when all your batteries are flat from the test!

My question is really, do you need an electrician to certify your emergency lighting to BS5266 every year?

There is no strict legal requirement to do so (as far as I'm aware) howevevEr it may be an insurance requirement (and I'm sure it's deemed "best practice" (or is that good practice, or even compliance?!!!)). I've checked and the "checklist" I referred to above is exactly what you state, confirming compliance to BS5266, this is an annual reinspection (starting on the 3rd year after isntallation) and we are requried to complete this as part of insurance policy. It's quite a common practice in my industry at least (retail). I don't know of a major that doesn't have this BS audit carried out annually (usually by a contractor - like we do).