The Tropical Fruit Forum

You might also try hand pollinating the flowers with a little brush JUST as soon as the flowers open.

A citrus tree drops all its flowers before fruit develops due to stress from environmental factors like inconsistent watering (both over- and under-watering), temperature extremes (like a late spring frost), or nutrient deficiencies, especially a lack of potassium. Other causes include poor pollination, insufficient sunlight, or a young tree needing to focus on root development. The tree is essentially shedding its blossoms to conserve energy and resources when it is under stress or unable to support the development of all the fruit.

Taken from google

Looks like damage from insects.

I wonder if a red colored grapefruit would turn red in the UK?  The red color in grapefruit requires a LOT of heat during the fruit's growth.

While grafting healthy they green scions is is always first choice, I believe you could successfully
graft the light green scions you ask about.

Frequently ones finger to test for moisture can cause problems.  The top inch of soil frequently is dry, while the deeper soil still fully wet.  A better method of testing is by lifting the container.

I doubt they would especially send anything to Russia.

Here are some of the most  common reasons for fruit drop.

https://www.arvensis.com/en/blog-what-should-be-done-to-reduce-the-phenomenon-of-fruit-drop-in-summer/#:~:text=%E2%80%93%20lack%20of%20some%20nutrients%20such,%2C%20zinc%2C%20calcium%2C%20etc.&text=%E2%80%93%20soil%20type/texture%2C%20especially,fruit%20is%20about%20to%20ripen.

Most all plants appreciate relief from 100+ degree temperatures plus the direct rays of the hot sun.  Be a friend and give it the  loving relief it needs, an the plant will thank you. 

I don't know the insect type, but they are insect eggs.

I have a large Valentine pummelo tree about 18 years old. The flesh of a valentine fruit is very deep purplish/red, and very sweet.  It has no (zero) acid taste,

Looks  healthy enough  I sure it will grow up into a fine tree under your care.

On a tree that small I would remove all of the fruit.   If however you wish fruit just leave one.  Citrus fruit draws all of its growth energy from only the three closest leaves to the fruit.