Plain shirts like a tshirt……..

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Roman, Next amongst others.

I have a few Roman tops, but the V is always too low for me, so I have to use a pin. I like their clothes, but sometimes they are very big, but not always, which is irritating.

Try White Stuff - it's online but there are also shops. They do tee shirts but also a variety of tops. I'm 5ft 2 and size 18 / 20 and have a number of really nice 'tops' , some plain and some patterned. Being short, I've had problems with v necks , but not from WS.
One of these days I might find out the reason for its name, as it has very little white stuff for sale, but a wide and varied range of colours. And amittedly not the cheapest, but reasonable, and they do have Sales.

Rowyn
White Stuff was founded in 1985 by George Treves and Sean Thomas. Starting under the name “Boys from the White Stuff” white stuff refers to snow.... Keen skiers, the pair decided to work together to find a way of funding their passion.
Wikki

I have a feeling this thread is going to be super helpful for me too !

Matalan have a few and they have 75% sale on now

Damart has some nice tops. I was pleasantly surprised.

Have a look at Kettlewell Colours for their Cara V, a short sleeved v neck T shirt in a variety of colours. I have several!
They’re currently in the sale.

Cotton traders have really nice clothes loads of tops good luck