"A small payload bay amidships"
From the Reaction Engines web site at http://www.reactionengines.co.uk/
"The SKYLON payload bay is 4.6m diameter and 12.3m long."
and designed
"to accept standard aero transport containers which are 8 foot square in cross section and 10, 20, 30 or 40 feet long. "
Note. RE does not *want* major UK Government investment. They have spent a fair bit of the last 20 years convincing UK civil servants that will *not* be required. Sir Humphrey's view (for years) was it looked like concorde, it would cost like concorde and it would be as uneconomic as concorde.
RE's goal is a *profitable* private venture *product* which can be owned by whoever can afford it. Buy it, fly it set up a satellite delivery service or just leave it on your lawn as a conversation piece (lawn size permitting).
BTW RE estimates their project would cost about £9.5Bn. This is an eye watering sum by private standards (unless you're a member of the Saudi royal family or similar) but is about what Airbus spends on a new plane.
One estimate of NASA's Ares/Orion project says it has consumed $17Bn *already* without even a single full stack being on the pad (Despite being canceled by the Obama administration the Senate appropriations bill still funds it to about 70% of its original planned budget while the rejected Congress bill wanted to fund it to 90% of its planned budget).