Apparently, UK experimental SABRE (Synergistic Air-Breathing Rocket Engine) ram jet engine is capable of single-stage-to-orbit capability at over Mach 5.2 (3,800 mph). SABRE engines burns hydrogen with atmospheric oxygen and when the space plane reaches an altitude of around 16 miles and five times the speed of sound (i.e. Mach 5), it switches to its onboard liquid oxygen tank to reach orbit. Being in orbit, such a craft can travel in opposite direction to the rotation of the earth thus saving energy & time of travel.
For example,
http://www.telegraph.co.uk/news/science/science-news/9709041/Britain-to-Australia-in-four-hours-with-new-engine.html confirms that using space plane travelling between UK to Australia could take around 4 hours.
Britain enters the space race as
http://www.telegraph.co.uk/business/2016/05/21/britain-launches-itself-back-into-the-space-race/ reported, “The Modern Transport Bill announced in parliament revealed the aim to “secure low-cost access to space for our world-leading small and micro satellite industry”, including helping drive through the complex legislation needed to certify space vehicles and their safe operation within UK airspace. A shortlist of five sites was revealed in 2015 for construction of space ports in UK : Newquay in Cornwall, Llanbedr in Snowdonia, and three Scottish sites, Glasgow Prestwick and Campbeltown, both on the west coast, and Stornaway in the Western Isles”.
R&D of future RAF space planes are also being conducted as
http://www.express.co.uk/news/uk/686046/Future-hypersonic-fighter-jets-fly-5x-speed-sound-leave-Earth-atmosphere reported, “Rapid response aircraft, as they are being called, will be equipped with technology similar to the Synergetic Air-Breathing Rocket Engine (SABRE), which is currently being developed by British company Reaction Engines Limited. BAE Systems has invested £20.6 million in the firm in the hope of seeing the next generation of hypersonic engine, which propels the aircraft to five times the speed of sound, installed in RAF jets. RAF jets of the future - UK warplanes will fight wars from SPACE to outpace enemy missiles. FUTURE British fighter pilots will fly hypersonic jets that are so fast they can outpace a missile and leave the Earth's atmosphere in order to reach their target within minutes.