Can I return my iPad Pro 11 128GB for a 256GB, because I find 128GB storage just not enough.

If purchased directly from Apple, you have 14-days with which to return your device for an unconditional exchange or refund. Beyond with days, you might consider a trade-in.

Apple Trade In - Apple (UK)

You should note that the trade-in value will be considerably less than you paid. You may achieve a better value by selling privately - and they purchasing a replacement iPad that is more suitable for your needs.

thanks

You’re very welcome. I hope it works out for you.