Llamas do well on poor grazing and can easily become obese on lush grass. However, mineral deficiencies in the soil due to farming methods in the UK can easily cause a problem and it is wise to have your soil and/or grass tested for specific problems. It is possible that selenium and magnesium will be found to be deficient but in the mean time salt and loose minerals should be supplied. Minerals in block form may be of use but llamas have to nibble bits from a block, being unable to lick easily.
In the winter hay can be used to supplement grass. Ad lib hay may be suitable for some llamas, e.g. the elderly, but can cause obesity. Barley straw can be fed ad lib to fill tummies without causing weight gain and to help provide heat in the winter.
Most llamas don't require concentrated feeds but pregnant or lactating females, crias and working stud males may require additional nutrition.