Odds for England if Parma got ashore
Elizabeth's force at Tilbury was about all that she and Leicester could scratch together. The key elements would have been her own guards, militia units such as the London Trained Bands, possibly the largely-ineffective levies led by the Justices of the Peace. Henry the Seventh had largely demilitarised the nobility of England, which explains why the earliest battles of the later English Civil War were such 'ad hoc' affairs.
Parma's forces were by contrast the most powerful army in the Netherlands, battle-hardened troops with experience in fighting the Dutch in the Low Countries. If they had got ashore, the Tilbury levies would have been outgunned, outnumbered and out of luck. Elizabeth had spoken of leading her troops, but it would probably have been Leicester who took charge in the actual fighting. Elizabeth would have probably lost London, the industrial areas of the Weald in Sussex, Essex and Kent. With such a substantial bridgehead in his hands, Parma would not have been possible to dislodge. Blockade of Parma by the English Navy was the only way to stop him thereafter.
Sidonia's forces were well enough equipped to have bottled up the forces in the South Coast ports, if the Duke had been as capable as the man he had replaced. Henry the Eighth's forts defending Falmouth and Plymouth were the only meagre defence other than the English Navy itself. However, it would not have been a walkover; men like Howard and Drake would have attempted to use their fast, heavily-gunned frigates to blast their way through Sidonia's force and reach the open sea. Galleons and galleasses would have had to almost form a wall to stop them.
My conclusion - albeit not well informed - is that Elizabeth would have escaped north from London, that some elements of the English Fleet would have escaped to harass the Spanish Navy, but that Parma would have been able to seize most of lowland England. Ironically, Elizabeth's one hope may have been Scotland; the Presbyterian Scots like John Knox would regard Parma as a manifestation of the Devil. They and the northern English would probably have made common cause behind Protestant Elizabeth, fighting in the hills against a Spanish Catholic occupation. It is also possible that Swedish, German and Dutch volunteers and mercenaries would follow the Anglo-Scots lead to preserve Protestantism.
The outcome would depend on weather - Parma's army might be as vulnerable to disease and cold as Napoleon's, probably losing support from English Catholics after high-handed requisitioning and abuse. Unless he could suppress the Scots and Northern English within the campaigning season, Parma would be facing increasing opposition and might have to withdraw.
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