Are you having a mammogram this time? Can't really understand from what you've said why this hasn't been done before. Perhaps related to you age?
If your appointment comes through for before your holiday it is, of course, entirely up to you whether you go or postpone it. I suppose you need to weigh up whether, if you postpone until after your holiday, will you really be able to relax and enjoy the holiday anyway. If you have the option to go to an appointment before your holiday it may all be OK and you can go on holiday safe in the knowledge that there's nothing wrong. I suppose when you ask the question "shall I wait till I get back" it all depends on what is more important to you - a speedy answer to the question you have about your health or how you may feel on holiday. Depending on the urgency your GP has attached to your referral, the decision may be made for you anyway, as your appointment may be more than 3 weeks away anyway.
There's never a good time to be waiting for/going to hospital appointments/tests. My breast clinic appointment was 2 weeks before a planned/booked holiday. We went along (my husband and I) to the hospital and then after the appointment (at which I was immediately diagnosed with invasive breast cancer) we cancelled the holiday which was definitely the right decision as it meant I was available to take a short-notice slot for the surgery. It was disappointing to lose the holiday but, on reflection, the least of my worries at the time. Our GP was supportive in signing all the insurance forms for free so we were hardly out of pocket.
Up to you whether you choose to tell your family/friends what you're going through. Personally I told my immediate friends and family beforehand that I had an appontment at the breast clinic and then once diagnosed I told a much wider circle of people. My mother-in-law kept her diagnosis to herself for YEARS and the rest of the family were left feeling very patronised, disappointed, left out, let down, insulted..... This was also the case with a friend who has teenage children from whom she kept details of her tests and treatment for several weeks, only telling them when she had to start chemo - her kids went ballistic that she hadn't told them and had thus denied them the opportunity to support her.
I wish you luck with the appointment, whenever it is, hopefully there will be nothing of concern found and you can enjoy the rest of the summer. Let us know how you get on.