Sorry, but you have not given us enough information yet.
The road base has to match the launching trolley, not directly match the boat. So the first step is to identify the launching trolley, and then find a road base to suit.
The West Mersea is an excellent design, and has spawned a number of near clones, including (I think) both Mersea (a separate firm) and Dixon Bate, and most certainly including Chris Brown (cbcoverstore). Some of these, perhaps most, are interchangeable; but some others (e.g. Snipe) appear to be dramatically different.
Assuming that the appropriate West Mersea base is suitable, I think (and I cannot conveniently check the point at the moment, as I am away and struggling with only a mobile device) that you need the 250 rather than the 220 road base. But check this point, via the details published on the manufacturer’s website or otherwise.
Hope this is helpful, although I realise it is a necessarily incomplete reply.
Oliver