A year or so ago I got some custom orthotics made privately. They were pretty good but did set me back £300 (seemed reasonable given the time that the specialist spent assessing me, getting them made, fitting them and then offering follow-up).
Some time later I discovered that my local NHS Podiatry Service has a specialist in musculoskeletal (MSK) issues relating to stance and gait. This is free of course, as long as you can get your GP to refer you (mine put me in the hands of Podiatry years ago because of my diabetes). I got a second assessment which was able to benefit from the experience of the first (private) one and the second set of orthotics are even better than the first. Because he wasn't 'just' making me orthotics he also spent some time on the underlying MSK issues and has given me exercises to try and stretch my calf muscles without aggravating the acute symptoms (some Achilles tendonitis and some heel pain, which seems not to be plantar fasciitis).
Based on my experience I'd say see if you can locate Podiatry within your local NHS structure and, if you can, see if you can persuade your GP to refer you to them.