At LGW BA had to purchase a purpose-built hi-lift Mercedes or VW (can't remember which) to service the RJ100 and ATR fleet. Whatever vehicle it is, it was extremely expensive and certainly beyond the reach of small carriers and small handling agents!
For WCHR pax, they disembark down the stairs and into a waiting mini-bus. WCHS and WCHC's need the special vehicle which presents its platform at the rear exit of the RJ.
Regional Jets always present a problem though, because the entrances/exits are very narrow - Thank God Rod Eddington's shipping them off to MAN and BHX (although I sympathise with their staff!)