I personally wouldn't go down the flash route as certain mobile devices (cough apple cough) won't display it and Android doesn't anymore, plus Adobe have discontinued flash for mobiles.
I would go the PHP route, with a custom system using custom templates.
I was involved in a project 2 years ago to build something similar in WP, using PHP to build the site.
The whole website cost in the region of £3k and took around 3 months to build.
It was for a photograph framer (you choose a frame, frame size, type, colour etc with a panel showing how it looks).
If you're interested, contact me via my website garethgillman.co.uk and we can talk specifics.
I am UK based like yourself