Sir82, they are, one of SBF's says pentecostal churches grew 22% between 1989 and 2005, proving evangelical churches are growing at a faster rate than the JW ones.
Again: the Pentecostal churches that are increasing are those that are made up of African immigrants.
The difference is that JWs are increasing overall, whereas mainstream Christianity is decreasing rapidly overall.
Of course when looking at the figures for mainstream Christianity it is always possible to exclude the declining churches, focus only on a few small churches that are being boosted by immigrants, and then proclaim - look other churches are growing faster than JWs! But it's a delusion. You might as well only count JW congregations that are made up of immigrants, ignore other declining congregations, and claim JWs increased 100% or 200% over the past ten years or whatever.
To get a bit of perspective here Pentecostals increased from 228,000 in 1979 to 287,600 in 2005, (according to the Church Census by Christian Research) an increase of 60,000. Over that same period the Church of England lost 800,000 at their services. So what the churches have gained through immigration has hardly made up for their losses. JWs on the other hand have had a net increase.
So yes immigrant Witnesses might be up a bit, and indigenous Witnesses down a bit, but overall they are still increasing. Contrast that with the small increases that a few marginal Christian groups have as a result of immigration which are completely dwarfed by the massive losses to mainstream Christianity as whole in the UK.