Bitsetting DVD+R, DVD+RW, and DVD+R DL discs to DVD-ROM is already enabled by default with the latest firmware from BenQ (I'm not positive, but I believe firmware revision B7P9 through B7T9 has this automatically enabled). But, there's also a software program that's free from BenQ called BookType Management v8.4 that allows you to pick the default bitset type for DVD+R, DVD+RW & DVD+R DL discs. You can keep them in their native formats, or change them to DVD-ROM using the BookType Managment v8.4 program from BenQ. Here's the link to download it: http://support.benq.com/files/Storage/Software/Book%20Type%20Management/BookType%20Management%20v8.4.zip
The BenQ can overspeed certain media straight from the box. But, use newer firmware (at least B7P9) to make sure you have more choices available for burning media at faster speeds than they're rated for. This can't happen with all media but only with certain media that BenQ supports. You'll have to use a utility like Nero CD-DVD Speed to see what speed your BenQ will burn each media type with. You can download Nero CD-DVD Speed here: http://www.cdspeed2000.com/files/NeroCDSpeed_361.zip
BenQ's B7P9 firmware (and any newer firmware) allows burning DVD+R DL burning at 4x speed for Mitsubishi DVD+R DL media (media code is MKM 001). I don't think there's support for any other brand of DL media for 4x speed burning. There are some other brands of DVD+R DL media that are supported for 2.4x speed burning. But, I believe that only Mitsubishi (MKM 001) DL discs are supported for 4x burning speed.
I can't help you concerning media in the U.K. because I don't live there. I hope this answers some of your questions. The BenQ DW1620 is a very good DVD burner and you will not regret purchasing it. Please look in other threads on this BenQ forum to find out what media types work best with the DW1620 and try to buy those in the U.K. I really like Verbatim 16x speed discs with media code MCC 004. But, I burn them at 12x speed because there are more errors when burned at 16x speed and it only saves about 30 seconds of time compared to 12x speed. Good luck with your new BenQ DW1620.