Do you have a particular need to show up at your final destination in denim? If not, I concur with what the other posters have said; wear something else.

For work (which is a situation where you don't want to be wearing lycra for any amount of time) and running errands, I ride in nylon cargo shorts (they look a bit dressier than athletic shorts, but don't have the same seam issues as a thicker fabric such as denim or even many khakis) or athletic shorts with either synthetic boxer-briefs (for sub 20 km rides-they dry faster) or a pair of the liner shorts (for longer rides when running errands). I change when I get to work, but it allows me to show up looking semi-presentable.

I have picked up all my padded liner shorts for less than $20, though I don't know what you can expect for sales on your side of the pond. Happy riding!