Crop consultancy Adas said that yields from the first UK wheat cuts were "lower than the five-year average", of 7.8 tonnes per hectare.
Some eastern areas, where lighter soils left crops more vulnerable to the spring drought, had reported "very poor" yields of less than 4 tonnes per hectare.
However, in quality terms, the crop was deemed "good" thus far, producing specific weights of 75-85 kilogrammes per hectolitre and protein levels of 12.8-13.1%.
And results from autumn-sown barley and rapeseed harvests, which are already some 70% complete, signal the wheat results may improve as combines fire up further north and west, where the drought hit less hard, and where many farms anyway have more moisture-retentive soils.
Adas edged higher its forecast for the average UK winter barley yield this year to 5.9-6.1 tonnes per hectare, from 5.6-6.0 tonnes per hectare, noting that "heavier soil types are tending to yield 8.5-9.5 tonnes per hectare for feed varieties".
For winter oilseed rape, which accounts for the great majority of the UK harvest of the oilseed, Adas lifted its yield forecast to 3.6-3.7 tonnes per hectare, from 3.3-3.7 tonnes per hectare,
"The yields for winter oilseed rape are good this year, currently averaging… about 15% above the national five-year average oilseed rape yield."
However, the consultancy flagged concerns over the quality of malting barley, noting that nitrogen levels were "often high".
The breaking of the drought prompted a late spike in nutrient levels in plants, as fertilizers which had lain inert were finally absorbed, meaning that they were concentrated in later stages of development than had been intended.
The concerns were echoed by Stuart Shand, sales director at UK grain merchant Gleadell Agriculture, who said that "the drought has really pushed nitrogen levels up", with "even the later winter barleys on the more drought-resistant soils trending higher",
"It is not uncommon for growers who normally get 1.60% nitrogen to have a shed full of 2.00% or over."
Brewers on continental Europe typically require nitrogen levels of about 1.85%, with those in the UK holding out for 1.75% and, for real ale, about 1.6%, with higher percentages risking problems such as cloudy beer.
Mr Shand added that, in yield terms, the so-called Null-Lox malting barley varieties, which produce longer-lasting beer, were "doing extremely well in comparison to conventional varieties".