Doing one's best for the DVLA visual field test requires a contradictory combination of concentration and relaxation. I have conducted many such tests, before the NHS decided it did not want multiple contracts and gave the contract to Specsavers. And of course experienced my own decline into loss of licence, my chin being ,on the chinrest! So, I hope the following will be helpful.
Make your appointment for a time of day that you feel at your best. Establish the location of the nearest toilet (if you are told there is not one for patients 'in store'). Arrive in plenty of time, fed and watered satisfactorily. Remove coat, scarf and any other clothing (careful!) to feel as comfortable as possible, and use the loo. You will need your driving specs for the visual acuity test. I despair at the requirement to use these for the VF test, theoretically separate reading specs are more appropriate for the working distance; request that you use these for a second attempt if there is any doubt over your success at passing having used distance or varifocal specs first.
For the test insist on the chair and the machine tablee heights being comfortable, no stretching or stooping. You should be advised that if you want a pause, keeping your finger pressed on the button freezes the process; you can sit back and have a wriggle, And just play with the button first, if you have arthritis it may be easier to use a forefinger rather than a thumb. Also helping is consciously blinking frequently (but quickly, to avoid misses) because this not only cleans the eye but revives retinal sensitivity. The need for concentration on the central target is obvious, but also relax your shoulders and shut out all other thought. Do not hold your breath! Breathe regularly and fairly deeply.
So many people report dissatisfaction with the test experience, I hope this helps to boost confidence a little.