But a World Cup is different. Of the three tyros who helped stun France only Rees is a certainty to be included in the final 30-man squad - by virtue of the fact he is the only specialist open-side flanker left in the camp.
Ashton wants to head for France with a squad packed with experience, strong leadership and mental toughness.
The England squad to face Wales contains no fewer than five Test captains - Phil Vickery, who will lead the team out, Jason Robinson, Martin Corry, Lawrence Dallaglio and Jonny Wilkinson - plus Farrell, the former Great Britain rugby league captain.
Ashton said: "My thoughts are that players being mentally tough enough to compete against the world's best and to live in that environment for five or six weeks is a very important pre-requisite for selection - possibly the most important of all.
"If you talk to champions of any sport they always put the metal side as the reason. Technically there is not a massive difference between players - but certainly the way players handle pressure can vary quite dramatically.
"You can see it in their daily life, how players react to people, who they look to for guidance. The ultimate test of it, though, is out on the field.
"This team is not short on leadership. We have five England captains in the squad and a man who captained Great Britain at the age of 21."
Farrell has yet to look fully comfortable at Test level but given his other qualities it would appear a solid performance against Wales today would go a long way to cementing his place in the squad.
Farrell, joined in the centre by the squat and powerful Dan Hipkiss, is up against a sizeable Welsh midfield in Gareth Thomas and Tom Shanklin.
The big lesson Ashton learned from the tour of South Africa was that England needed to become more physical.
As a result, giant lock Simon Shaw has a great chance to seal a World Cup place while Ashton has gone for a huge back row of Corry, Joe Worsley and Nick Easter.
"I am excited about what I have seen over the last couple of week and by this bunch of players. We are not even half way through our preparations yet. We still have another five weeks until the tournament begins. There is still a lot of work to be done," said Ashton.
"If we can translate into action what we have been talking about doing, then England may well be in a position to surprise a few people when we get to France."
The Rugby Football Union have pre-sold 64,000 tickets for the game against Wales. Tickets will be available on the gate but the RFU advise spectators arrive early.