For Garmin (and other brands that use the same format) the .csv file is just a list exported from a spreadsheet with the Latitude, Longitude and a name for each of the entries. Obviously you need to have them in the right order and have the exact format of headers etc just right, but it's all pretty simple. For TomTom .ov2 format, AFAIK, you just need to plug the data into a converter program that takes in .csv and squirts out .ov2.
There are a few free ones around on the web.
As you're UK based, one suggestion I'd give is to contact PocketGPSWorld.com and see if they would be interested in a collaborative effort?
They already have some files for low bridges in the UK and some other countries, but I don;t know how complete they are.
I know their POI Library manager may be interested, and he has all sorts of clever tools for making usable files out of raw data.