Before the UK joined the EU, there was no way for IR mark owners to overcome objections based on similar earlier rights (similar to the present situation in the US). Nowadays, however, the UK trademark office only notifies the IR mark owner of such similar earlier trademarks. It will neither refuse protection for the IR mark in the UK nor will it inform the owner of these earlier marks. Therefore, IR mark owner does not have to take any steps upon receipt of such a notification.
There is no need to spend time and money to reply to notifications from UKIPO to IR mark owners citing earlier third party marks. Such notifications can be ignored.