What is the relationship like between Hammerfell and High Rock ?

That depends. The question of "when?" is really important here, as is "who?".

Hammerfell and High Rock have more often than not been divided entities and not unified nations. High Rock, despite looking like a unified country under the Empire as the 'Province of High Rock,' is actually divided into many petty states warring with one another. Unlike Skyrim, for example, it has never really had a High King. Hammerfell on the other hand has a tradition of a High King, but the province has been bitterly divided by Forebears and Crowns for most of its history. This division is so deep that the two factions rarely helped each other even in the face of common threats.

Like I said, "when?" is really important too. At various times, the greatest states of the Iliac Bay have allied with one another, such as Sentinel and Daggerfall's alliance during the 1st Era siege of Orsinium (although Daggerfall and Sentinel would be at each other's throats before the war even ended) and during the Second Era where Wayrest, Daggerfall, Orsinium, and Sentinel cooperated to fight against The Ebonheart Pact and Aldmeri Dominion. At various other points, they've fought. The Ra Gada of Hammerfell tried invading High Rock in the 1st Era, and Sentinel and Daggerfall warred over Betony during the Third Era.

Building off of this, weren't the Crowns and Forebears somewhat reconciled after the Great War? "In Hammerfell, Imperial fortunes took a turn for the better. In early 4E 173, a Forebear army from Sentinel broke the siege of Hegathe (a Crown city), leading to the reconciliation of the two factions." - http://uesp.net/wiki/Lore:The_Great_War_%28book%29

Anyway, per the Redguards and Bretons. They're not outright enemies, but they're definitely unfriendly neighbors most of the time. Well, except for the prelude to Daggerfall's plot with that whole War of Betony thing- they definitely didn't like each other then. But you won't see Redguards and Bretons hating each other within the games, compared to the Dunmer/Argonian or Nord/Altmer relationships.

The relationship between England and France might be a good anology?

Did we help, Pharaohs_?

True. First time too since the death of High King Thassad II in 2E 862. At least as far as we know.

Thank you guys this was a lot of help.