Barnaby Street/ Oxford Street have sale on now the cheapest suit is around £90; otherwise try Oxfam shops/ car boot sale (marché au puce) are all around London the most expensive is about £25.
Since you said suitS, me thinking you are planning a serious shopping trip ... If you live near the south, why don't you drive one weekend to Andorra and do your yearly shopping ... everything is some 30% less.
Sorry if this does not help ... But you never know