Are the political parties in Britain and the USA becoming 'catch-all'
parties and why?

I don't know what is meant by 'catch-all' but here in Britain the
parties all have similar policies and they are marketed like soap
powder. Beneath their labels, they are very hard to tell apart.

Why? It's part of the 'democratic process': give the population and a
cosmetic change of Government every few years and they will be happy.

And why shouldn't the electorate be happy?

Generally, we are doing OK but, of course, the politicians themselves
are doing even better!

Becoming?  Haven't they always been?

http://www.answers.com/topic/catch-all-party

Since the US and British electoral systems require candidates to get
50+% of the votes (simplified definition), they have to appeal to a
broad spectrum of the populace in their voting district.  This means
that candidates and parties that have a narrow, idealogical platform
have little chance of getting elected.  Right-wingers, Liberals,
Socialists, Greens, et al. don't have much of a chance, but on the
other hand, the major parties have to attract their potential voters,
so they are receptive to these voters' interests.
If you will, the platform of a catch-all party, by defining prior to
an election what principles and projects it will pursue, is similar to
the coalition agreement in other countries, where smaller parties with
individual, narrower platforms have then to agree which of each
other's primary interests will be the basis for the coalition.
People and many (but not all!) political theorists on the continent in
Europe feel the the British and US systems deny representation of
narrower minority interests.  Maybe this is true, but I don't think
that there is any evidence that these political system result in
better government.

Does that help you?
Myoarin

there are two kinds of electoral systems.  proportional representation
and regional representation.

proportional represenation means you divide the seats in the
legislature according to how many seats every party or candidate gets.
 this is much more fair to smaller parties that can be represented
with as little as one seat in the legislature.  there are still large
mainstream parties, but they might have much less party discipline
among the members as there are far more opportunities to switch
parties or even run as an independent.

america, britain, canada and many others still use regional
represenation, where each candidate needs to secure a majority in a
particular riding.  this will almost guarantee a narrower choice for
voters because there is virtually no way for any fringe groups to get
elected unless they can concentrate their votes in a particular
riding.  i'm not sure how it works in britain, but here in canada it
gives tremendous control to the party leaders that can control the
nomination of candidates that will stick to the party line.