The expression <я пошёл к себе> is in the past tense but it means "I'm off home" which is the present tense. Why is this?
For reasons unknown to me, verbs of movement with prefix "по-" are used in the past tense to indicate intent.
In general, prefix "по-" means the beginning of an action when used with any of the following verbs of movement: бежать, вести, везти, гнать, ехать, катить, лезть, лететь, нести, плыть, ползти, тащить, идти.
As a result the following phrases are used as farewells:
Я пошёл. / I'm leaving.
Я побежал. / I'm leaving in a hurry.
Я поехал. / I'm leaving by car, or public transport.
In a similar way the following phrases mean the intent of starting an action in the immediate future:
Я понёс. / I am starting carrying [it].
Я покатил. / I am starting rolling [it].
Usage of verbs in past tense for that purpose is unusual. A popular Russian writer K. Chukovsky says in his book "Alive as life itself":
Или эта форма: я пошел вместо я ухожу. Человек еще сидит за столом, он только собирается уйти, но изображает свой будущий поступок уже совершенным. С этим я долго не мог примириться.
Or this form: "я пошел", instead of "я ухожу". The man is still sitting at the table, he just started contemplating his leave, but he expresses his future action as complete. For a long time I could not accept it.
пошёл has perfective verb aspect - verbs with prefixes usually do. It refers to a one-time event of starting to walk. If you're walking right now, that event of starting to walk is already in the past.
шёл would be the corresponding imperfective aspect, referring to a habitual or continuous process of walking in the past.
I would say it's akin to Present Perfect in terms of English grammar, because the verb is in perfective aspect, i.e. the act has been performed and its effect is still being felt at present. So into English it would have to be translated as
I have gone to Seva
The question has probably been misunderstood because of a spelling mistake, so the response is "off home".
Provided the Russian here is Я пошёл к сеБе (not севе) i believe one cannot meaningfully compare these two sentences because their structures are markedly different, it's only fair to compare more or less grammatically equivalent constructs.
Russian does have a construct similar to I'm off home, and that's Я домой, whose tense is equivalent to that of the English expression.
With a verb this idea can be expressed in all 3 tenses with a varying degree of determination (in a descending order)
Я пошёл домой
Я иду домой
Я пойду домой