Of course there are alternative views:
http://www.migrationwatchuk.org/brie...t+of+migration
The problem is the the facts are unclear and whilst there is no doubt that a proportion of migrants directly contribute a great deal financially to the economy, there are many who do not. These may be asylum seekers who (through no fault of their own) are unable to work and therefore pay no tax and migrants whose earnings are so low they pay little or no tax.
There is no mention in the articles you quote of how these conclusions were arrived at by the TUC and until HMG gets to grips with the situation and produces credible migration figures there will always be serious doubt as to what the real situation is.