There are lots of things to be considered before even contemplating taking on large contracts. If it is a national contract, how will you execute it, both technically and logistically? Will you be able to do the work in the time frame with your existing equipment, or will you need to invest in new equipment, a TM, for instance. What about staff costs, NHI, holidays, sickness? Are you prepared to be away from home a lot? Can you finance it? Would you survive if the client goes bust? Would it be a good thing to have almost all of your eggs in one basket? Will you be able to meet the H & S requirements without spending lots of money? Will you be able to live within the terms of the contract? And are you prepared to have your working life dicated to by the terms of a contract?
I always wanted to be in the commercial sector but started off small with a national contract for a clothing chain that was not actually a contract, just a working agreement over prices etc, but nothing in writing. We grew as they did and kept them for many years whilst building up other commerical clients. We tend to avoid tenders as they are only ever won buy the lowest bidder, someone prepared to exchange high profits for volume, or even prestige.
If I were you and you set upon a career in the commercial sector then start off small because you have a great deal to learn and the only place to learn it is on the job and if you f up you don't have so much to lose.
Hope this helps.
Simon