Jo,
I don’t know, probably not an NHS candidate because that’s an average length and they tend to be interested in micropenis. I’m sure you’ll be able to get it done privately if you want but I’d talk to your GP about getting a referral to a specialist. It’s more likely than not that your GP will attempt to float you down the body dysmorphia route and even if he/she doesn’t the specialist might. Tell them you are going to fly to mexico on a bigger penis holiday and they might see you as being serious but the important thing is to get your GP to actually refer you appropriately rather than fob you off with immediate counselling from a generic (local) psychotherapist.
I don’t know (and haven’t looked at your post history) whether you started on big weights but I think some people simply fuck up their chances by going down routes that allow the penis to adapt to the stress. If you’ve never done girth work, have you considered trying it to see if you can stimulate length gains?