Can we bring lavender wands from the US to the UK and back again?

As you are bringing agricultural products into the US, they will need to be inspected by the USDA staff at the airport you arrive back at. If there is no USDA staff on hand then the CPB officials may do an inspection on their own or more likely simply confiscate the items for destruction.

I have no firsthand experience with UK import rules on dried flowers and straw, but would imagine they are equally strict on importing agricultural products.

It looks like you can probably bring 1 wand per person in to the UK. The rules are disccused broadly on gov.uk here, with the specific restrictions documented in PLANT HEALTH CONTROLS ON PERSONAL IMPORTS OF PLANTS AND PLANT PRODUCE AND PRODUCTS, which says:

From all other non-EU countries

• 2kg of fruit and vegetables, and
• cut flowers and any parts of plants together forming a single bouquet (maximum of 50 stems), and
• five retail packets of seeds (but not seeds of potatoes).

There is a warning that certain plants are further restricted, but as lavender is grown in the UK, I would not expect it to be subject to such restrictions.

With regards to returning the lavender to the US, this however looks like a complete no go. According to the US CBP:

Travelers can check the general admissibility of fruits and vegetables by consulting APHIS' Fruits and Vegetables Import Requirements (FAVIR) database at www.aphis.usda.gov Select the type of fruit or vegetable in the "Approved Name:" field, then select the country of origin in the "Country/Region:" field.

You will receive one of the following results:

• 0 entries found means the fruit or vegetable is NOT allowed into the United States

• x entry(ies) found [followed by the name of the commodity and the name of the country]. Click on "CIR".

  • If the import requirements indicate: 1 Subject to Inspection : This commodity is subject to inspection at the port of entry and all general requirements of 7 CFR 319.56-3. The fruit or vegetable is allowed into the United States pending inspection.

  • If the import requirements indicate: Condition of entry treatment then the fruit or vegetable is NOT allowed into the United States in passenger baggage.

Lavender is not on the list at https://epermits.aphis.usda.gov/manual/index.cfm.

Note all of these rules seem to be imagining live plants - I couldn't find any clarification for either end with regards to dried.

Practically, lavender is available in the UK if you really want it there, and it's probably not worth the fuss. (Equally, it seems unlikley that either end will be tearing apart your bags because of a suspicous floral scent, but it's normally best not to lie/dodge customs because of all the trouble you get into if/when they do pull you up).