Strange really as the standard solos are still selling well, wheras the SRs never have. Only three operators around here have them, Arriva Derby bought a batch back in 2009, yet last year bought a batch of standard ones, Nottingham City Transport bought a large batch of SRs last year, and Marshalls of Sutton on Trent near Newark have 1. I think Solo sales in general are down though, as the economic situation is meaning many smaller operators who run Solos are feeling the squeeze, so are not replacing older models, and bigger companies are tending to drop the sort of routes that Solos run on. Then again, I can't see how only offering the more expensive SR would boost sales. Problem with the SR is not only do they cost more, weight more, but also lack a lot of common parts with the standard model and require more expensive curved glass windows wheras the Solos are flat. Perhaps they feel they can take advantage of a captive audience with the DDA deadline looming and quite a few none compliant buses (including early Solos) to replace.
Regarding the Streetlite, from what I've heard they are not good at all. Then again the company I'm most familiar with that has them bought pre-production versions so they did have a lot of teething issues (worst being leaky windows that then leaked onto electrics, so basically if it rained they would just die and need towing back).