It's a bit like why in the German version of Terminator, they didn't get Swartzenegger to do the dub. The reason was that in German Speaking countries the Austrian accent (which Arnold had) was seen as an accent farmers had. A bit like the Deep Southern Accents in America.

In the UK, the west country accent is seen as being the accent of Farmers, and also being the accent of slightly thick country bumpkins (no offense to anyone from the West Country).

In the US and the rest of the world the West country accent is linked with Hobbits in Lord of the Rings (like Sam and Merry's accent, Frodo had a generic English accent and Pippin had a Glaswegian Accent, perhaps due to the fact Billy Boyd was Scottish), but in the UK, the west country accent is seen as the accent of country bumpkins. I believe (however I can't confirm) that is why Stephen Merchant was chosen to be the voice of Wheatly.

Different accents have different reputations, it's why in adverts different accents are used for different products. For life insurance ads, they use Scottish actors, because that's linked to integrity. For DIY products Cockney accents are used because it's linked with tradespeople, for things like oven chips and bread, a yorkshire accent is used because that is linked with home and "good, honest simple food".

I would like to see a threatening Bristol accent, but I don't know how it would've worked.

Also, here's a video of Prowse in the vader outfit doing the lines.