But the OP also has a bigger mortgage due to the offset. It's not the same as having extra 'spare money'.
Imagine that some time ago three families moved into the same street and all bought houses for the same price and with a mortgage of £200k and all needed some work. All had the same income and outgoings, situation all the same.
Family 1 cracked on with it, paying for their new kitchen and the other work on the house as they went along, together with paying off their mortgage in the standard repayment manner. They've now finished all the work, have a lovely modern home, not much in savings, say £1k, and owe £150k on their mortgage. They've also spent any spare money on 'living for today' so have had new cars, holidays, eaten out a lot etc.
Family 2 have also done all the work to their house, and decided that they will use any spare money they have to overpay their mortgage, so they only owe £50k on their mortgage. They have a bit of savings too, but less than £6k.
Meanwhile, the OP decided to take out an offset mortgage and build up some spare capacity in the offset pot before doing the work on her house. She's nearly there when coronavirus strikes and she loses her income. The OP is the main earner in the family, so this is a big hit.
Her mortgage balance is still £200k, but she has £100k in her offset pot. She is therefore the only one out of the three families effectively expected to risk the security of her home, solely due to the type of mortgage she has.
If the OP had spent all her money on doing the house up, buying and doing things or overpaying her mortgage, she would be entitled to universal credit as would families 1 and 2 above. But because of the way her mortgage is arranged, she isn't and rightly feels aggrieved at the unfairness of the situation.