they will be either 1/4 or 3/8 it would be better to get a new tap that is better suited to low pressure hot water as just changing the hozes will not make a huge difference if they are 1/4 thread as it goes into the tap, a 3/8 thread should be just about ok but the best would be a normal tap or a two hole mixer as they have 15mm / 1/2 conections but that probley means a new sink ?
Find out what size you have first and like i said if 3/8 try new hozes if not a new tap but get one with 3/8 holes and 15mm pipes if still not good enouth then its new sink ? and 2 tap hole mixer or two single taps this is what it should have had from the start, then that is the best its going to get with out going for a pressure system