There's no specific problem with having a TV which can receive both analogue and digital signals. If you're planning to use the set in African countries, however, you need to be aware of the different broadcasting standards which are used in those countries.
TV signals in the UK (both analogue and digital) are broadcast using a system called 'PAL I'. TV sets designed to receive these signals MIGHT be able to receive signals broadcast using other PAL systems (e.g. the PAL B system used in Mozambique) but you should check this before purchasing a TV set for use in countries which use other PAL systems.
Many countries in Africa (particularly those which used to be ruled by France) use various versions of the SECAM system. It would be extremely unlikely that any TV bought in the UK would be able to receive these signals.
You can see which system is used in each country, here:
http://kropla.com/tv.htm
Chris
PS: Remember, as well, that voltages might vary between countries.