Assuming that you're only interested in reading out the bingo numbers or quiz questions, or amplifying your sermon to your congregation, rather than pumping out full-blast dance music, you can get away with a simple domestic set-up.
Find your way to your nearest BHF Furniture & Electrical store:
https://www.bhf.org.uk/search/location-search?servicetype=furniture-electrical-store
and look for an amplifier from an old 'hi-fi separates' system.
e.g. like this
http://www.hifi-pictures.net/amplifiers-home/Sony%20TA-212/amp.jpg
or this:
http://audiovintage.su/images/hifi/vollverstrker_196.jpg
Also look for some speakers to go with it. (Check that the speaker connections match those on the back of the amplifier or that you know how to get them to match up). You might be looking at around £30 expenditure in total.
Then get a cheapish microphone from somewhere (£10 to £20 in Maplin) and you're ready to go.
Alternatively, if you're lucky at a car boot sale, you might get the lot for a tenner!