First off, congratulations on being one of the very few people who can spell Middlesbrough correctly!

I'd second UK flyer's advice and add that the Tube at Heathrow can look scarily crowded, but it's usually quite easy. Use the credit card machines instead of queuing. Then go down the stairs, turn round and go to the very end of the train. There's almost always space there. And if there isn't, walk over to the other platform - these trains leave every 2 minutes, you're not in trouble if you miss one.

I assume you're going to Middlesbrough for family/friends? It's not exactly on the tourist routes... That said, it's got some excellent industrial heritage (the transporter bridge, the civic buildings), some wonderful countryside around it (Rosebery Topping and the North end of the North York Moors) and some great coast (Saltburn by the Sea, or even further afield to Whitby, for the best fish and chips in the country). It's about the most industrial place I've ever been to in the UK (mostly due to the massive expanses of British Steel Redcar and ICI Wilton) but it definitely has its spots.

Enjoy your trip.