My twopennyworth.
From a servicing point of view the B ain't a 'Specialist' car in terms of exotic machinery. We drive a rebodied Fiat Punto, bog std mechanically that could've been bought for £10k brand new in Germany. Fairly Low Rent. Just above a Kia in the Food Chain, perhaps. No whizzo engine bits needing highly skilled care or highly paid & trained specialist Fiat mechanics. Bog Std. Made out of rusty recycled Indesit washing machines. Never a Premium vehicle that can command a high resale price where plus or minus 10% in value is MONEY.
On another issue Main Dealers don't waste time displaying cars that are much more than 3 years old either. They farm them out to the little man. In comparing a oldish B to a newer posh brand BMW on resale ain't juggling eggs with eggs. That's all my rude bit.
The savings made if the B had been 'little man' serviced will compensate for any possible loss of future resale value which sadly has gotta be lowish. What's the potential max percent loss. A little of not a lot.
So in answer to your question imo you'll lose virtually zilch by using a Little Man opposed to a Main Dealer when servicing.
There are Bs circa £2K for sale on Ebay. The B is affected by the credit crunch too. Now if your car was worth 15k to 20k and above premium and newish, different scenario.