When my son worked for UA we did it to/from LHR in summer a few times. My son was able to check the loads to see the best days to travel. One time we waited from 7:00 am until about 2:30 to go to any east coast city or ORD. Then we were offered seats to SFO (10 hours in middle E- seats). We took the seats as we thought it better to get out of LHR. We spent the night in SFO and connected home to IAD the next AM. This is one of only about 3 less than great experiences flying standby on UA, and we used the privileges constantly for 6+ years. But as relatives we had better priority than the companion tkts, and we could do a lot of segments if necessary to get us out of perdicaments.

It is important to remember that if you get stuck, buying a tkt home and/or paying for hotels/meals while you wait out a backlog could cost you more than buying a tkt in advance. My son's sister-in-law once had to buy a full Y tkt home from Japan because she had to get back to work. You have to weigh your risk tolerance before you leave home.

We found that May, Sept, and Oct were good months to standby to Europe. Good luck.