It will not be a gardenia as they are white and not hardy in the UK. It could be a camellia from the description. If so the reason it has not flowered is that you trimmed it. Camellias need no pruning except to remove any dead bits. The flower buds are formed the year before flowering so by trimming it you cut off next years flowers. Leave it well alone and during the summer you will see fat buds appearing at the ends of the branches. These are next year's flowers. LEAVE THEM ALONE! If it is too big for its sitation it would be better to move it than prune it. Google image camellia and check if that is what you have.

My camellia needs pruning or it takes the whole garden over. It can easily grab an extra 4 sq metres a year if not pruned back.

However, pruning never stops mine flowering. Maybe it is just small ones that are affected by pruning? Or time of pruning? I prune in march taking all the branches which have no flowers on. Then late April a mass of new shoots emerge from further down the branches and it is these which bear next year's flowers.

I think you have hit the nail on the head, rubble. You prune in March just after flowering and giving the plant time to recover and form buds. If you pruned in the autumn, the time many people feel they should be 'tidying up' their garden, you would lose your flowers for next year. I would still maintain that an unpruned camellia is more natural looking if you have the space.