Wales' hopes of retaining the Six Nations crown are all but over after suffering back-to-back defeats against Ireland and France.
They boast just one victory from their opening three engagements, beating Italy 42-0 in their opening match, and will hope to get back on track in their final two outings.
A trip to Twickenham beckons for Wales on Saturday and new head coach Wayne Pivac, who admitted it may take time for his philosophy to set in, is boosted by the availability of Williams and Biggar.
Williams hasn't played competitive rugby since sustaining an ankle injury at the World Cup in October, while Biggar picked up a knee issue as Northampton lost to Saracens last weekend.
The duo have been given the green light from Wales' medical team and go straight into the starting XV. Pivac has omitted Josh Adams, Taulupe Faletau and Gareth Davies as Tomos Williams and Josh Navidi return.
Rob Evans replaces Scarlets teammate Wyn Jones at loose-head prop as Wales look for a first win at Twickenham since their 28-25 triumph at the 2015 World Cup.