Should railways outside the UK routinely offer discounted return fares, funded by raising the price of single fares? For the most part, there is very little difference between singles and returns in the UK, meaning that going there and back is cheaper than it would be if there were only single fares. I realise there are some discounted return fares outside the UK, for example the weekend return in Belgium, but in general there is usually no discount over two singles.
In a lot of countries, point-to-point fares
aren't charged according to how much the market can bear, like in Britain; instead fares tend to be priced to be 'fair'; what you appear to be proposing is that railways abroad move towards a more market-based pricing regime.
To some extent, this is already happening, but
not by the introduction of cheaper return fares
per se, but in two quite separate ways:
Firstly, yield management techniques are commonplace on High Speed services even in countries which previously had purely 'distance based' pricing models. Secondly, for traditional regional/urban services, many countries have a range of products which allow multiple journeys to be made very cheaply, such as area-based tickets, which can pay for themselves with just one return journey.
For example when I travelled from the Czech border (near Cheb) into Germany, I found that it was cheaper to buy a Bayern area ticket for 3 of us (total 45 euros) than just one single ticket each; we then decided to do another journey, which we'd otherwise not have done.
A big problem with comparing Britain to other European countries (apart from: which one do you choose!) is that you are comparing a country that is obsessed with charging the maximum possible amount that people are prepared to pay, and is keen to price demand off the railways, with countries that - generally speaking - offer very reasonable fares, which are priced in order to be affordable to the average person.