I've found FindMyPast.com very useful. All sites have a basic free service but it's going to be very limited and if you're serious you are going to have to pay to get access to some serious database muscle. You can use it to build a very detailed tree. If a line goes off-piste (to France, for example, in my case) its difficult to pick it up
Ancestry.com is quite good if you strike a good line and want to chase it back a long way quickly. Ancestry is the biggest and if somebody else has hit that seam you are in luck, although to do it properly you'd have to validate everything.
It's all a matter of luck really. Some counties have better records than others. If you have roots in Devon you may have real problems because a lot of Devon parish records were destroyed in the wartime bombing of Exeter cathedral. Cornwall's records are pretty good but a big chunk of mine were from a part of NE Cornwall that used to be in Devon at one time, so those records are lost.
Of course, if you are looking for Scottish relatives just come to the Mitchell Library in Glasgow. It's not open at the moment but it will be eventually and if you love records you'll be in heaven.