There are a few things to bear in mind:

The Blackwall tunnel is very busy during rush hours in the morning Northbound and evening Southbound. I'ts not so much the Blackwall tunnel, but the approaches that get congested and you can wait hours to get into the tunnel. Best to close your windows once in there - the air is bad.

The tunnel is often restricted at night during weekends - there are warning notices on all the major A roads of closures weeks ahead.

tip: if you are coming from South London and want to go East to Lakeside etc on the A13 take the left turning after you leave the tunnel - very easy to get wrong since your intuition tells you London is on the left.

The Woolwich Free Ferry has been in operation for a long time and is vulnerable to breakdowns. There is more than one ferry boat but it is not unusual to find only one operating. The queues round the Woolwich roundabout can get very long and block up the other traffic. Watch out for information about closures as you approach and head off to the Blackwall tunnel (direction of Greenwich) if there are long queues.

There is a foot tunnel for foot passengers (who could also go on the ferry for a free ride).