Tell us about your 12 miles a week.
While your current bests indicate a high level of talent in terms of being a "monster" you may be a little short.
Wilson Kipketer ran the same 800m time as you at the tender age of 13. Kevin Sullivan ran it at age 14.
I could list a hundreds of guys who ran faster 1500ms than you have run at age 18.
I would assume you are doing lots of quality training like 10x400 , 4 x 600m , some gym / conditioning work etc.
You are now faced with a dilema:
I think you are pretty close to your ceiling in terms of what you can achieve with a quality only approach.
If you try and add more quality training you will most likely get injured or mentally burned out so you now need to change the whole way you train.
You built the roof before the house.
Bottom line is the top guys are doing heavy volume with heavy quality at exactly the right balance. If you want to elevate to that level you need to correct your current balance and add signicantly more volume.
Being based in the UK I suspect you have subscribed to the Coe approach which makes a lot of sense in terms of the quality work except everyone forgets and coe never mentions that his volume work went unrecorded as his so called "junk miles".