It's an offence under Regulation 103 of the Road Vehicles (Construction and Use) Regulations 1986 to "cause or permit a vehicle to stand on a road so as to cause an unnecessary obstruction".
If you were 'ticketed' for that offence, and decided to challenge the prosecution, it would be up to a court to decide the definition of 'an unnecessary obstruction'. However (in my opinion) it's likely that any 'awkward' parking, which caused another driver to have to exercise additional care to that normally required, would be classed as 'causing an unnecessary obstruction'. (The court would probably consider the analogy of 'obstructing a public footpath'. If you place something on a footpath, so that people have to go around it, you're 'obstructing' it, even though people can still move freely around the object. Simply 'creating extra difficulty' seems to be enough to count as 'an unnecessary obstruction').
Chris