An accident can be investigated and fault attributed to one or another party, or it may be agreed on a 50/50 split of blame. In this case any legal expenses claim will also be reduced in line with the percentage split.
So, for example, if you claim £1,000 for out-of-pocket expenses and the accident is determined to be 50/50 then your insurance would only cover 50% of your expenses - in this case, £500.