Unless you are living in Spain or a Spanish national, borders are closed at present (10 May 2020). As JCaron pointed out, nobody knows whether it will be worse or better in July. But for today:
These are the people who are exempt from border restrictions:
Air, land and sea borders
Only Spanish citizens or those who can prove residency in Spain by ...
Regarding France, while you may technically be able to enter the country (if you are an EU/EEA citizen or resident), you are not allowed to travel inside the country except for specific listed purposes (professional travel, funeral…) Going on holiday or switching residences is not allowed.
The rules are set to change tomorrow (May 11) and short trips inside ...
As other answers have told, in the current situation the rules will change as a result of the spread and control of Covid. The more controlled it is, the more lenient you can expect the rules to become.
I will just explain the current status in Spain so you can make your own evaluation.
As of now, travel between Spanish provinces is restricted to essential ...
Since the dissolution of the Border Force NICE unit, there's no Border Force unit that liaises with the general public.
Per TIMATIC, the database used by airlines:
Visa required, except for [...] making a landside transit with a confirmed onward ticket for a flight to a third country that departs before 23:59 the next day. They must [...] have an ...
The UK rules, as show on the official UK check if you need a visa website state that you can transit without a visa with a landside transit (going through immigration) on the next day if:
you arrive and depart by air
have a confirmed onward flight that leaves on the day you arrive or before midnight on the day after you arrive
have the right ...
Does it mean that if I get to London at 18:30 On may 22, I can leave before 23:59 On may 23 without needing a transit visa?
Yes, that's exactly what it means.
I have an F4 immigrant visa, does that qualify?
Yes, an F4 immigrant visa is a US visa. It qualifies.
I cannot answer the others.