Edmund of Langley, Duke of York was acting as Keeper of England. His main qualification for this role was undoubtedly his rank, but he had performed the task on two other occasions without any apparent problem. Richard II has been faulted for choosing Edmund but that's surely hindsight speaking as he was the obvious choice, being the King's senior male relative.
As late as 20 June York's government paid a sum of £1586 to two of Bolingbroke's squires, towards the £2000 a year allowed to Henry in his exile. (That's an annual salary of more than £800,000 modern money. Some tyrant that Richard II, eh? Right up there with Marshal Stalin!)
Anyway, the point is that the English government's intelligence service, if it had one, failed miserably on this occasion. Henry's preparations to invade were well advanced by this time, and by 28 June news had clearly reached York as he wrote to various sheriffs asking them to bring armed men to a rendezvous at Ware. On 4 July, or thereabouts, Henry landed at Ravenspur, having also arranged an attack on Pevensey Castle as a diversion.
By this time York was writing to bishops, nobles, knights and esquires ordering a general mobilisation, and by 12 July some seventy knights and squires had responded with their retinues. Among them were king's retained knights such as Hugh Despenser (Thomas Despenser's cousin) and William Burcester (who was lined to the Despensers by marriage.) At least five sheriffs brought in their posses. Magnates to show up included the earls of Wiltshire and Suffolk and the Marquis of Dorset - John Beaufort, Henry's own half-brother. Several bishops turned out, including Henry Despenser, Bishop of Norwich, a man more noted for fighting than praying. The estimate for York's army is between 2000 and 3000. Small but potentially useful. There was plenty of money available to pay recruits. Richard's Exchequer at this time was full of cash, and indeed the King had gold stacked in barrels up at Holt Castle.
Edmund of Langley had a respectable war record in France, and his standing up to bullying by Gloucester in 1388 showed that he didn't lack courage in a cause. However, when it came to generalship he was no Robert E. Lee or Oliver Cromwell. More like Gypsy Rose Lee or Oliver Hardy. He marched his men north with the obvious intention of interdicting Henry's advance. Then he stopped, dithered, and marched west. Away from Henry, leaving London and Westminster wide open. The best interpretation is that he hoped to join Richard when the latter landed in Wales.
(Main source for this post is The Royal Household and The King's Affinity by Chris Given-Wilson. A specialist work, but very useful, and highly recommended to anyone interested in this era.)