Some time ago I experimented with an *cough* unofficial offsite backup of the TopoGB maps on my eTrex. My conclusion was that bitmap maps were poorly suited to such a device, on account of the lack of map resolution, slow CPU speed and inability to scale text to fit the small display (especially when not used in 'north up' mode, which is necessary for the text to be the right way up). I decided that it was likely to be useful for walking, where the quality of the terrain mapping of OS maps can't be beaten, but pretty rubbish for (even off-road) cycling.
Instead, I purchased a full set of OS maps in the ViewRanger app, which I could happily scroll around quickly on my significantly more powerful (and power-hungry) tablet and smartphone, sticking to vector maps on the Garmin. As they were Android devices, I could plot GPX routes using the OS map from the comfort of my tent and transfer to the eTrex with a USB cable, without need for an internet connection.
Which worked perfectly until Viewranger was borged by OutdoorActive...