If I am living and working in the UK on a work visa, are there any restrictions on how often and for how long I can return to the USA for either a holiday/leisure or work related visit? I've been told by a friend you cannot be back in the US for more than 28 days in any given calendar 12 months but that seems unreal? I am a US citizen.
Secondarily, I understand that with a work visa I can only perform the job for which the visa was granted within the UK. Does that in any way prohibit me from returning to the USA for brief freelance work performed as an individual, outside the UK company I would be employed by?
You can travel/ be out of the UK for as long as you like, provided it is part of your annual leave or travel for work. You cannot take a large period of unpaid leave from your employer or anything like that.
However, if you are looking to apply for ILR at some point in the future (Tier 2 General??), your absences must be under 180 days per 12 month period and your employer has to provide a letter which includes lots of info, some of which says that all absences were on annual leave or for work travel.
Nothing stops you from taking work while in the USA, provided it is taxed appropriately. This would not contravene UK immigration rules, as long as you are not 'working remotely' from within the UK on a Tier 2 General visa.