"As ever" is similar to "always" or "as they always have been". Eager as ever to do something means always wanting to do it. The politicians are eager to join in and criticize or punish BP because they always want to do want they think the nation wants from them. (The Americans are mad at BP, so the politicians want to show they are too; they have the same opinions about BP, are "tuned in." To keep the boot on his neck is to keep them under control (think of a police officer or soldier with is boot on someone's neck to keep him on the ground).