I am a UK citizen living abroad & would like to buy an apartment in London. 

I would like to find a reputable company (or if necessary companies) that would:

1. Help me find a suitable apartment. 
2. Help me get a mortgage. 
3. Rent out the apartment & manage the rent so that I can cover all or
at least most of the interest payments from the rental income.

I would also like to find a company that would give sound unbiased
advice on property investments in the UK.

Ideally, I would like to be able to rent it out to a student or an
expat who would leave it in the summer for a few weeks and would be
willing to allow me to use it in return for a reduction in rent.
A less desirable alternative would be to rent it out for most of the year (11
months) so I could use it in between rentals.

I am self employed in Egypt and also have a source of income in the UK
as I earn commissions from a UK based company.

Just a small input on part of your question:
It may be a good idea to rent out to students, maybe charging per
room. This way a 2 bed appart would give you a better rental and would
leave it free during the long summer break.

I can not give much help with the other part of the question other
than my recent experience in buying my own home.

I used a broker to organise the mortgage. You could look on the
internet for the right appartment and a good solicitor will be able to
converse with you while your over seas.

An estate agent will help get you student rentals, but I do not know
of a complete service provider.

Hope this helps some.

You have three main choices:

a) let to a group of students on a 9-month rental with the property
void during the summer vacation. You could then use the property
yourself or let it on short (weekly) summer holiday lets. Because
students leave at the end of their degree anyway there is usually no
problem regaining possession at the end of the tenancy. (Do not let
out to students per room - you will have a lot of hassle with
individual tenants coming and going. Let only to a group where all
members of the group are jointly and severally liable for the rent and
dilapidations.)

b) let on short assured tenancies which are between 6 months and 2
years. During that duration the tenant has security of tenure and the
right of exclusive occupation of the property. If the tenant is
awkward, recovering possession may be time-consuming and expensive.

c) let on corporate / holiday lets. This will likely attract the
highest rents but will also have higher amount of void periods and
higher overheads, as you will have cleaning/caretaking charges. The
property also has to be furnished and maintained to a very high level.
Location is very important and many clients will at the top end will
demand a portered building with secure garaging.

A lot depends on what type of property and where you will be buying.

If you buy a flat in London (or anywhere in England) you need to be
aware of restrictions and costs associated with leasehold property.

I suggest you peruse the Association of Registered Letting Agents (ARLA) website at
http://www.arla.co.uk/
and 
http://www.arla.co.uk/btl/index.htm

ARLA membership is usualy considered to be a good indication of a more
reputable business.

Do you have to buy in London? Rental return on investment has fallen
in some areas of London because the market has been flooded with
buy-to-let. Other areas have more affordable buying prices and produce
better return on investment from a rental income point of view,
although perhaps less capital growth long term .