If you've just changed your mind and there's no reason for you to be unhappy with the product itself (they have followed your instructions for the customisation and haven't misled you as to what it will look like) then you're stuck. They have no obligation to accept the return.
If the item isn't as you expected and they've done something wrong then you may have more luck. Ultimately they've given you what you asked for and they're now unable to resell it as they've customised it.
Slightly different then, ive changed my mind because its not entirely what I expected now I've seen it in person, which I guess it why the online 14 day thing exists. its not faulty though, just not what i expected
Ah. In which case you're arguing with them over whether the item (with customisation) was not as described. If that's the case then they have to accept the return, but you'll have to convince them it's not as described.
If you've just changed your mind and there's no reason for you to be unhappy with the product itself (they have followed your instructions for the customisation and haven't misled you as to what it will look like) then you're stuck. They have no obligation to accept the return.
If the item isn't as you expected and they've done something wrong then you may have more luck. Ultimately they've given you what you asked for and they're now unable to resell it as they've customised it.