Riff and others,
Perhaps I can help once more (but do please let me know when I stop helping).
ITARs - it's not quite true that ITARs mean that foreign customers 'are not provided' with 'the best avionics/radars etc available'. ITARs have been around for years, and their purpose is to closely control what military technology (and data, and related services) are provided by US companies to what foreign 'persons' ('Persons' can mean governments, companies and individual people). One of the key ways ITARs control what US companies can export is via things called Technical Assistance Agreements (TAAs), which take around 6 months to a year to put in place.
Remember as well that, unlike the UK, the US DoD generates and owns most of the validation and verification data that supports bringing aircraft and weapon systems into service. (Stuff like flight test data, environmental test results, EMC testing, and so on). TAAs don't cover DoD owned stuff, so strong and comprehensive 'Government to Government' agreements are essential.
ITARs and other US regulations can be incredibly restrictive, but the UK has been generally successful in getting access to 'top end' US military technology. The best way to ensure success is to make sure that any UK purchase of US equipment is backed up with a solid set of Government to Government agreements, such as Treaties (e.g. the 'Bermuda Treaty' that supported Polaris and Trident) or Memorandums of Understanding (MoUs) like the 'STOVL JSF MoU' that made us a Tier 1 partner on F-35. These not only allow Government to Government data transfer, but they also provide essential support to US Government approval of TAAs. As ever, politics plays a big part in how comprehensive these agreements are and how much technology transfer they support. That's why Israel always gets a good deal, as does the UK.
However, (there's always a however) you cannot ever take this sort of situation for granted. When people in the UK think that our 'special relationship' means that we can still get a good 'off the shelf' deal without getting these TAAs and agreements in place, they get a rude shock, and our defence programmes suffer. That happened on Apache, it may have played a part on Rivet Joint, and it sure as hell happened on many of our UORs where we wanted rapid purchase of US sourced kit.
In the case of F-35, the UK is very well covered across the piece, and I expect that as a Tier 1 partner, we will get very good access to the data we will need. Will we get everything the US has? Probably not. Will we get what we need? My assessment is 'probably yes'. But it will require hard work.
Best regards as ever to all those working the detail in the documents,
Engines