It's been my experience that chipping and breaking tile grout is the surest indicator that the subfloor wasn't properly prepared. It was probably laid directly on the wooden subfloor rather than a cement board like Hardibacker (here in the U.S.).
Having said that, there are several brands of flexible grout such as Uni-Bond available. I'm sure if that brand isn't available in the U.K. another will be. Just ask your favorite DIY store for examples. You're likely to have problems with it from time to time but the overall results will be better than what you have now...
Best of luck!