In my work you can claim 10p/mile if you cycle on business. This does not cover cycling to work, nor does it apply if you have a C2W bike as technically it belongs to Council (until end of agreement).
then you need to fill in P87 to get the extra 10p per mile you are allowed to claim for business mileage. assuming you use your own bike and not a CTW bike or a bike belonging to your employer.
the standard rate is 20p per mile with AFAIK no upper limit on miles.