LONDON, March 30 (Reuters) - Britain's biggest retailer Tesco TSCO.L will increase the price it pays for all its fresh milk by nearly 20% as suppliers battle unprecedented levels of on-farm inflation, it said on Wednesday. Several of Tesco's own brand milk lines, including four-pint variants, are covered by a scheme which matches the prices of German-owned discounter Aldi. On Tuesday, market researcher Kantar said UK grocery price inflation hit 5.2% over the four weeks to March 20, the highest level since April 2012.