I used to be scared of heights and have overcome heights up to the first floor. I wouldn't be happy fixing guttering on a two storey house though, just bungalows/stables, etc.
Nothing to be ashamed of though. Although it's different, all the builders I know are too scared to do plumbing (unless under instruction from a plumber!)
I've done my Part P electrics but am still too scared to put in a socket or new wiring for an electrical shower.
If I don't like a job (heights, electrical or whatever) I won't take it on unless I know of someone who can do the stuff I'm frightened of. For example, if it's a bathroom (upstairs) and the soil pipe has to be replumbed to exit a different area of the wall, I'll ask my builder mates to come and do that aspect for me. Ironically, it's probably cheaper for the customer too!
So no, it won't hinder you becoming a plumber but if you want to be a retained fireman then that's a different matter!!
Also, and I'm sure you're aware of it, your and your customer's health and safety must be a priority. If this means renting scaffolding because you feel unsafe working from a ladder, don't skimp. You only get one life. 50% of all injuries in construction are from people falling off ladders. Your life is more important than trying to save the customer a few pounds from not hiring a proper work base.
If it's solar heating what's wrong with hiring a couple of builders to put the panels on the roof while you do all the connections? If the builder charges £160 a day, you can always tack on another £20 for the customer as your fee for finding a reputable builder.
Also there are not many jobs in the plumbing world requiring ladders. Most of the time (when starting out) you're going to be squirming around the floor with a cricked neck trying to undo and tighten tap nuts under the bath and kitchen sink.
