the 18 week "clock" starts from when the hospital receives the referral from your gp. There is only one thing that can stop it is if you are not physically or mentally ready for treatment (eg if you went to pre op clerking and they found you had a heart problem that needed sorting before a hip op) if you are only at an xray stage and havent even seen a consultant yet, your "clock" hasnt started yet. you may find you dont even need a hip replacement, or are not suitable becase of osteoporosis or something else, so i would recommend waiting and getting more info first.
With free choice, you can look at infection rates of all the hospitals too and chose by lowest rate. ps, do research: dont just belive what you read in the papers re infections