"We don't know any of these hymns" ... please choose some we do know.
In the dim and distant past when I played piano (and sometimes even organ sans pedals) for small churches with no other musicians I was often given the hymns literally five minutes before the service. Whether I or we knew them did not matter, my task was to get through them the best I could. Latterly, I had a minister who'd arrive having chosen nothing and say 'I think we'll have open worship today' - i.e. the congregation call out songs and you play them. I would not wish to return to those days, I think it is common courtesy to let churhces and musicians know at least a few days in advance what the hymns/songs will be. I am not anti 'open worship' but it is nice to know more than five minutes ahead that it's coming!
For all that, there was an acceptance that if you invited a speaker he or she was free to choose their own hymns and our task was to sing them (I got quite adept at changing tunes). There was no way we'd have told the visitor to choose another set of hymns - that would have been utterly rude. I think it still is.
Manners aside, the need to sing things we know speaks to me of making church comfortable for us - a place where we know what to expect and can have a nice time. Not knowing any of the hymns puts us in the place of the person entering church for the first time or who does not speak our language - we find oursleves excluded, uncomfortbale, not knowing what to do. I think it is good for us to experience this once in a while as it keeps us alert to the foreigness of church.
Baptists are good at criticising more liturgical traditions for their 'vain repetitions' of set prayers, and can even be wary of the Lord's Prayer and 'The Grace' becoming little more than formulas. Yet in some churches a repertoire of half a dozen songs each sung 37.5 times is not questioned - can these too become vain repetitions? I think they can.
What I have learned from this latest experience of hymn choosing is that the good people of Dibley are to be applauded for their willingness to sing whatever I throw at them. My pianists both know that they are free to alter tunes, and the couple of times a year that members choose all the hymns gives them the chance to get their own back! We fairly regularly learn hymns/songs that are new to all of us and even sing in French, Latin, Spanish and Xhosa on occasion. We might be seen as stuck in the dark ages, but in no way are we hymn ignorant!