Here are some tips for travelling from Cambridge to London. Prices fairly accurate in April 2015.
National Express operates a service between Cambridge and London. It’s not a well-used service, chiefly because it takes a good two and a half hours. However, note that if you don’t go all the way to Victoria but instead jump onto the Underground network at, say, Mile End, you can slice off the last hour or so. It’s also worth noting that 10-trip tickets are available (only from the driver). These work out as £5.65 for each one-way journey, so they are cheaper (and quicker) than buying advance tickets online, but they do not guarantee a seat (not that getting a seat has ever posed a problem for me).
Consider taking the train to Liverpool Street (LST) rather than the default King’s Cross (KGX). There are “slow” and “fast” trains to both destinations, with the fast LST trains taking about 70 mins compared to the fast KGX trains taking about 50 mins. Although the LST route is a little slower, it has a number of advantages. First, it is much less crowded. Second, it is noticeably cheaper: if you travel with a Network Railcard after 10am, you can get an day return (which includes a Zone 1-6 London Travelcard, and has no evening peak restrictions) for £16.75; the same journey to KGX costs £20.80 (with Travelcard) and you cannot return between 1630 and 1900. Both the LST and KGX routes use trains of varying quality; the LST route has both the best trains and the worst. Those providing the 1004 and 1021 service to LST, and the 1707, 1737 and 1807 services back to Cambridge, are the best trains, replete with large tables, comfortable seats, and free (if rather intermittent) wifi.