Ian, that sounds like a wonderful project, albeit bittersweet about the closure.
To add some comments to the already good advice here, I have a few suggestions that might be helpful.
Agreed on PanF being contrasty, but you can tame it with a dilute metol-only developer like Perceptol. I've had wonderful results with this combo in sunlit snowy winter scenes, and never had a problem with blocked up highlights or shadows. Wonderful tones too. But as Skellum points out, reciprocity is a real problem here if it's dark in the church. And if you're not familiar with using the PanF-Perceptol combo, then to stick with a combo you do have experience with is great advice.
As there could be some high contrast light in there, a softer film and developer combo might work really well. I've had great results shooting indoors in old buildings and barns using HP5 in Perceptol 1+1. I think the rich, soft tones that this combo gives could really suit the old architecture of your space.
Time of day for shooting might be a big consideration as well. If you want to balance the exposure of indoors vs. outdoors -- say if the windows are clear and the surrounding landscape is worth including in the images -- then maybe choose to shoot in the early morning or late afternoon/evening when it's not so bright out. Likewise if there are some special stained glass or window features you want to retain, then avoid midday sun streaming in a window or those will be all blown out. Then again, this is the UK right...forget I mentioned the sun...
As for colour film, Portra 160 and 400 have lovely warm colour balance, which might be great for indirect daylight to counter the bluish cast. The P400 especially so...P160 is a bit more neutral. Overexpose these by about 1 stop to get the best tones and colours. Ektar 100 is very finicky and not forgiving of exposure errors, especially overexposure, so I would avoid it if you've never used it. The Portras, OTOH, are very forgiving and have lots of exposure latitude. Wonderful films, if the colour balance works for the scene. I don't think any chrome film will have the exposure latitude you need, so go with negative in colour.
Hope this helps -- any more questions on the above, just ask. Best of luck!
Edit: I highly recommend taking a spot meter, if you have or can borrow one. You will likely have a lot of deep shadows and brightly-lit areas, so nailing exposure will be a lot easier with a spot meter. Or just bring lots of film and bracket everything :-)