Armitage was voted the outstanding player in the Top 14 last season and has continued that form into this campaign, producing another eye-catching display in Toulon's Heineken Cup victory over Montpellier at the weekend.
Delon Armitage, who followed his brother to Toulon in the summer, scored two tries in that match and appears to have recaptured the form that first saw him selected for England.
Lancaster, who will name his autumn international squad next Thursday, will not formally close the door on the English players in France.
But the fact they are not available to join up with England outside of the official international release window counts heavily against them.
"What makes it difficult when a player plays in France is they are not available for our camps when we need them," Lancaster told BBC Radio Five Live.
"We have got a camp coming up at the end of this month (at St George's Park) and we have an agreement with the Premiership clubs to have access to the England players.
"It is not under the IRB release window so we can't get the lads who are in the French clubs.
"Toulon were in the French final the day we played South Africa in the first Test (in June) so it creates problems from that point of view.
"You would never say never but it is always going to be harder for players playing in France."
The Rugby Football Union's policy on the selection of Top 14-based players is that it would only happen in exceptional circumstances and Lancaster believes he has more than enough options at his disposal.
Steffon Armitage is seen by some as the only ball-winning openside flanker available to England, but Lancaster and forwards coach Graham Rowntree disagree.
"Over the summer tour Chris Robshaw stole more ball than any other seven playing in the summer, including New Zealand and Australia, but he is also a credible lineout threat and a ball-carrier," Rowntree said.
When players move to France they are well aware of the implications it can have on their international careers.
Any British players involved in this season's French final will be unavailable for the British and Irish Lions tour because of a clash of dates.
"The players that go there, go there in that knowledge," Lancaster added.
Another English player back in form is Harlequins number eight Nick Easter, but he does not feature in England's long-term planning.
"Nick Easter is playing well," Rowntree said. "We are well aware of his form but since this coaching crew came together, post the World Cup, we decided to look a bit younger, the likes of Ben Morgan, Thomas Waldrom, Phil Dowson have been playing eight for us.
"We are quite happy with their development because we have to look at the next World Cup.
"The door is not closed for Nick. He knows that. Age is not so much counting against him but the guys we have got there we are very happy with.
"If there is an injury, Nick knows he is one we will consider."
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