Can a priest announce that on Good Friday there will be an alternative service in the neighbouring Anglican church in addition to the one held in our own Catholic church earlier that day? I have no problem with ecumenism, and have no problems with Catholics attending such a service, but surely that should be seen as additional, not as an alternative. Are there any rules or guidelines on this?
It all depends.
It seems from your post that the 2 services will be at different times.
Anglicans (non-Ordinariate) cannot be admitted to Holy Communion; whether that is at a Mass or at the pre-sanctified on Good Friday.
My U.S. version of the Book of Common Prayer says that “communion” is optional at the Anglican Good Friday service. I would presume that the same applies in the UK. Naturally, Catholics could not ever participate in that (be present, yes, but not participate).
This depends on exactly what’s happening.
Currently, on Good Friday, the Passion readings, veneration of the Cross and Communion form a single service that must be done as a whole. Catholics “should” be participating in this, although it’s not a Holy Day of Obligation. As far as that goes, the priest should certainly not be telling people “come here or go to the Anglicans, it doesn’t matter.” It matters a great deal.
It’s also traditional to pray the Stations on Good Friday. I would have no problem with telling people that they could go to one or the other, with the understanding that it could never be done in such a way as to imply indifferentism.
This sort of thing can get very tricky. The difference between true ecumenism and indifferentism can be very subtle when it comes to relations between Catholics and Anglicans.
Based only on what you posted, it’s not clear if what’s happening is acceptable or not.