Sensible move imo. The only surprise to me is they haven't trimmed capacity elsewhere too. Better to cancel with a few weeks notice rather than the trauma of finding out at the airport.
More and more crew will be through training, more aircraft available and hopefully the airports will be better by the time the end of June rolls round.
It's still been incredibly poor planning and prep from tui, but they have also fell victim to the problems out of their control. Maybe it's also time they took a leaf out of their main rivals book and looked at taking more control and responsibility in house regarding ground handling. MAN/BHX/LGW and possibly BRS are big enough bases to warrant it and see the benefits.