Can I (US Citizen) legally travel to France and Back right now? (late May 2020)

No, you cannot enter France unless you have a good reason, and holidaying currently isn't a good reason.

Any person entering France must (currently) fill two self-certification forms:

• one that specifies the reason for entering France, from the given list
• one where the person certifies they don't have any COVID-19 symptoms...

Additionally, a 14-day self-quarantine will be required from June 8th upon entering France (and another one when returning to the UK).

The details and forms are available on this page (scroll down for the English version and the links to the forms):

1/ In the context of the implementation of the state of health emergency in France because of the COVID-19 pandemic, access restrictions to Mainland France and to French Overseas territories are in effect.

Every traveler is requested, since 8 April 2020, to fill out and carry one of the following travel certificates according to his situation :

• For an international travel from abroad to mainland France
• for an international travel from abroad to French overseas territories
• for a trip from mainland France to French overseas territories

These documents are made available below.

From 25 May 2020, travelers are also requested to fill out and carry with them a statement certifying they do not have any symptoms of a COVID-19 infection. The statement can be found below.

The certificate and the statement are to be presented to transportation companies before boarding, as well as to border control authorities.

2/ From 25 May 2020, the following sanitary measures will apply for international travelers upon arrival on the French territory :

Travelers coming from the outside of the European space (all countries except European Union member states [except Spain], Andorra, Iceland, Liechtenstein, Monaco, Norway, San Marino, Switzerland, Holy See), as well as travelers coming from the United Kingdom (from 8 June on) and from Spain (by air only), will be invited to carry out a 2-week quarantine at home, or in a dedicated location provided if needed, upon arrival in France.

The list of allowed reasons is:

• French nationals, accompanied by their spouse and children;

• European Union nationals and nationals from Andorra, United Kingdom, Iceland, Liechtenstein, Monaco, Norway, San Marino, Switzerland, and the Holy See, having their primary residence in France or transiting through France to reach their country of origin or where they have their primary residence, accompanied by their spouse and children;

• Third country nationals, who are holders of a French or European residence permit or valid long-stay visa, having their primary residence in France or transiting through France to reach their residence in a country of the European Union or assimilated;

• Third country nationals, transiting less than 24 hours in an international area to reach their country of origin and who are holders of a travel document to this country;

• Diplomatic mission staff, or international organisations staff working in headquarters or offices located in France, accompanied by their spouse and children;

• Healthcare workers supporting the fight against Covid-19;

• Flight and cargo crews, or travelling as a passenger to their departure base;

• Foreign nationals ensuring the international carriage of goods;

• Goods carriers including seamen.

None of which seem to match your situation.

A few things are supposed to change in the next few days, details should be made public the afternoon by the french Prime Minister. It is however unlikely people from the UK will get additional flexibility at this time.